Manganese (Mn) immobile would be my guess.

Red & brown spots on the tops of the leaves and maybe a red streak on the stem or stock as well?

What is the humidity around your plants?

Try making a foil spray with about 5% Epsom salt and give them a good soak when the lights are on. Add some Epsom salt to your water too about 5-ml per gallon works.

If you do have a dry environment that will cause additional transpiration and in turn cause more manganese to be drawn into the foliage.

That's just a guess from what you are saying.