Interesting....If I were to go with just the pictures, I would say it is getting too cold at nite for the plants, but your temps say no. How about Cal/Mg or some unsulphered molasses?

I had an Acorn that did that every single grow. As stated before it could be just the species, which I am not familiar with. I do not think it is anything to really worry about, those are healthy leaves. I would let the grow finish without any more nutes.

Are you sure the pot temps are not dropping below 50F at night?