Wow! your plants really blew up since the last time I was able to check in.
Looks really good so far. If all of the plants are the same strain, I would think maybe the one showing mag deficiency is a different phenotype that is just a bigger hog when it comes to magnesium. Try foliar spraying it with mg when the lights are off if you want to isolate it and see if the new growth shows signs of improvement.