Ok heres what I have found in a nutshell on the magnesium deficiency thing so far, and its vague in whats available for info so for anyone trying to figure it out maybe this can help point you in a direction.
It seems that if potassium levels get too high a magnesium deficiency will arise, haven't figured out why yet sorry, and leds don't cause the deficiency but intense lighting will magnify it and make it more apparent sooner.
I wish I could give you the "why" for this but I haven't found that info yet.
Is it possible your K ingredients are on the high side
@Keffka ?
That being said, exposure to intense lighting should tell you if your mag ratios are solid.
This could explain both the LED conundrum and why when your raising your vpd it arises more often, as most lower the light to raise vpd resulting in intenser light.
They are both cations, so possibly too much K attracts and holds what Mg needed to bond with??
Also at PH 6.0 and below, Mg uptake in soil plummets quickly, so adding microbes to raise the ph a bit could in theory fix it if your pots have become too fungal dominant.
Any thoughts?