It definitely looks like a calcium deficiency. Thats pretty typical for a SIP with LOS. The fix is easy, the explanation takes a bit longer, but once explained it makes sense and becomes easy to manage. But it's calcium, and Gee, so go grab a coffee. This will be a long post even by my standards, and will barely scratch the surface, but all you need on calcium is the basics, so here we go....
If calcium is incorrect, nothing will work right. Think of calcium as the battery that powers the grow. If the batteries get low, everything slow/stops. If the batteries get overcharged, everything fries.
Calcium is a heavy mineral so it constantly sinks. Calcium is also your soils primary electrolyte. Magnesium is also an electrolyte. Calcium and magnesium need to be ratioed somewhat correctly for calcium to be able to run the soil AND electrically keep magnesium in it's place. Hence CalMag was created. Don't knock it or turn your nose up at it, it works. Period. But if you need it with your soil mix, you should start adding it regularly from the start.
So when calcium is dominant over magnesium, magnesium sits on it's hands and plays nice (its a ruse, magnesium is an a$$hole).
Magnesium is actually nature's bodyguard. When calcium gets low on the ratio, magnesium gets high in the ratio. For every excess molecule of magnesium in the ratio, a molecule of nitrogen gets locked with it. Nitrogen is the main driver in protein synthesis. Magnesium is electromagnetically sticky, and if calcium isn't present to neutralize magnesium's charge, it sticks to things. Nitrogen is it's 1st choice. It's a 1 to 1 lockout ratio. It stops protein synthesis.
So if calcium is low, magnesium steps up as the primary electrolyte and does it's job, which is to shut down the grow by locking out nitrogen. It does this to leave the nutrients safely in the soil until calcium returns, and nature is happy again so she lets a plant grow there once more.
So what happens in real life is you are growing away and the leaf damage you see, appears. So you add CalMag, it balances out the ratio, magnesium has to let go of nitrogen as calcium is back, and electrically forces magnesium to sit on it's hands again. CalMag fixes magnesium issue this ON CONTACT, that's important to remember.
So when calcium gets fixed, you get a huge rush of excess nitrogen all released to availability on contact. I would suspect your leaf curl came after a calmag feed or a top watering. There are various ways of fixing this.
@StoneOtter runs LOS and SIPs very well, and has a calmag schedule/technique so hopefully he can weigh in here.
Myself, I recycle my soil, so I don't like adding CalMag as it builds up over time, so I choose to topdress EWC and top water it in.
This whole scenario is why I tried the bottom watering thing and quickly went back to cloth pots and top watering. Not saying you should ditch SIPs, just that it needs to be managed differently. I didn't want to drastically change my system that already works for me.
So in a nutshell, your soil is likely locked by magnesium, which means nitro is locked, so you will likely see a nitro def next. Adding calmag from the top will fix this, but all that nitro will get released at once, and you will see more leaf curl, but better a nitro rush in late veg than in flower where it will screw your crop yield.
You need to burn this nitro off before flipping. After you fix calcium, you need a regular supply of it, either through the res (Stone can guide you here) or in a top dressing form that needs to be watered in from the top, not the res. I use EWC and top water it in. EWC works like calmag, as it's loaded with calcium, but it's far more nutritious than calmag. It also negates the need for teas, but it requires top watering.
Fix calcium before you try teas or any other feeds. Feeding 1st only make your pile of locked nitrogen larger, you don't want that.
The easiest way to keep an eye on calcium in LOS is with a refractometer or your fingers.
With a refractometer (analog, not a digital one) your reading line will be crisp and sharp if calcium is low, and fuzzy and blurry if calcium is correct. So a weekly check keeps you ahead of calcium. Then you learn the calcium feed schedule your grow requires.
With your fingers, you can let the pot dry to the point where the surface goes completely dry for at least an inch deep, and see if the dry surface has gone crusty and lumpy. If it has, that is excess magnesium in the soil locking out air and tying up nitrogen. CalMag will fix that, but it's heavy, so it's going to move down and will need to be constantly managed.
So yeah, it's a calcium deficiency. Your Rev's mix, which I also use, is heavy in dolomite lime. Dolomite is a natural supply of dry calmag. But it will likely go deficient too, right about the time stretch finishes. Thats a terrible time for a nitro overdose.
I have worked hard with my growing style to avoid this whole situation, so I'm not an expert on the fix, I choose avoidance. DO NOT OVERFEED CALCIUM. It's not one of those things where your gut feeling of adding a bit extra will help. It will hurt. Follow the mixing instructions, and 2 or 3 light doses work better than one heavy one.
Be warned though, because not only is nitro locked, oxygen is heavily restricted too, and every speck of food that a plant eats MUST be attached to an oxygen molecule for the system to recognize it as food, so you may have food laying around everywhere but not enough oxygen to activate it.
I tell you this because on top of all that nitro that will cause some tip curl, you may get an overfeed too, causing brown tips. Be prepared to see that.
But your still in veg, so if you fry a few leaves you can grow them back before flip. After flip, the leaves you have are all you get for the most part, so fix it now while you can recover in veg.
Refractometers are cheap. 20-30 bucks for an analog one.
Like carbon, most people misunderstand calcium too. Most think of it as a nutrient, and it is to some degree, as every cell contains it, but what it really is, is a soil conditioner. It sets the stage for all the other actors to be able to play their parts.
It doesn't matter how good the actors are, without a stage no one gets to see the show. Microbes run on electricity. Calcium supplies this for them too.
Think of your soil as a stack of plates in the cupboard. 12 plates stacked and the stack is 12" tall. Now flip every 2nd plate on edge. The stack is now 6.5 feet tall. That is EXACTLY what calcium does to the soil. It changes the magnetic properties of the soil particles and instead of everything magnetically sticking together, every 2nd molecule pushes instead of pulls.
Think magnets. stick 2 together, now pull them apart, flip one over and put them back together. Now they don't stick, they repel.
When calcium and magnesium are in balance, the charge in the soil becomes perfect for every 2nd plate to stand on edge, literally. Repelling. It's called tilth. (Tilth is worth googling, just to understand it a bit so you are aware of it)
Now the soil fluffs instead of compacts, air rushes in (78% nitrogen, see where this is going?), and an aerobic environment gets created. The stage has been set.
Thats a lot to digest. All you really need to know is you have to have calcium, but its heavy and keeps sinking out the bottom, so you need to constantly add more from the top. (Or follow Stone's SIP fix). Then get ahead of it and stay ahead. If you use too much, magnesium gets completely neutralized and your soil turns to dust. You don't want that either.
A house plant is a great way to experiment with calcium. If you have one, let it dry out a bit and see if the soil is hard and crusty once dry.
If it is, give it a dose of calmag, let it dry out, and check the soil again. See how many applications it takes to make the soil good again and watch it's effects on the plant. DO NOT OVERDOSE CALCIUM. It's literally electricity in a bottle. Things will fry.
So, the silver lining....
Once you get your head wrapped around calcium, your whole game will go up at least a level, but really, if calcium is good you will be able to compete with the best of them. It's the single most important thing in your soil.
There's a perfectly good reason why with almost every problem presented, you see someone say "Add Calmag". Thats because if you don't ensure it's correct before you fix something, the problem will quickly return. In veg, no big deal. In flower, well the clock is ticking so yield will be compromised.
Don't let all this scare or intimidate you, it's just background info as to why/what is happening. If you never learn more about calcium beyond the fact that you need it for some mysterious reason, thats A-OK, as long as you know you need it.
Here is how important it is. If N-P-K was there as a system to tell you what a plant needs to survive, not what nutrients it needs, it would be Ca-N-P-K. Ca would be the 1st number, it's more important than food.
Hopefully you finish this read before you die of old age
And again, don't add too much at once, and don't let calcium intimidate you. Just make sure you have enough consistently, and you’re golden.