Lockout? Deficiency? Disease??? HELP

So a little over a week ago I transplanted my three plants from dixie cups to gallon pots. I know 200% that I did NOT disturb the roots. They are 7 1/2 weeks old in FF Ocean Forest soil and haven't gotten any nutes at all.
Two days after the transplant I noticed one plant that the veins on the lower leaves started turning brown. After another day or two the lowest leaves started curling up and turning a grayish brown. Around that same time I noticed a second plant with veins starting to turn brown like the first one. My first train of thought lead me to believe that maybe the FFOF soil that has been sitting for only three had bad fertilizer in it causing low pH and nute lockout. Even if that wasn't the case I would assume that fresh soil, especially FFOF would have plenty of nutes to prevent deficiencies.
Three days ago I flush the first plant with a gallon of 6.5 pH water and yesterday I flushed the second plant. At first the flush seemed to slow it down but now it hasn't seemed to do anything. From everything I've read over the past week it seems to look most like Mg def to me. My next step is to either flush it again with 1/2 tsp epsom salt per gal if its lockout/Mg def OR water with some FF nutes if its just a plain old deficiency.
I'm hoping its not a disease or fungus masking as a deficiency.
I'm no expert with only a few months of weed growing experience, but I have been growing palm trees, fruits, and veggies for many years and I've never had a problem that I haven't been able to correct until now. I really need to fix this now bc my third plant that is going to be a mother is very VERY slowly starting to so signs of the same fate. I have a 5 month old plant that is two weeks from harvest that was grow the same exact way and I never had any major problems with that one, definitely nothing like this. If this continues I'm gonna have to cut my losses and clip as many good clones as I can.
As for the pics, the first two pics are the first plant to show signs, and the third pic is the second plant showing signs.

I'm not looking for a straight up answer, just some opinions. I have to have missed something at this point, maybe someone here can figure out what that something is.