It looks like you may have the beginnings of Magnesium (Mg) deficiency. If your leaves are also curling or twisting, then that would also make me think that.
Try adding one level tablespoon of Epsom salts per/gallon of nutrient solution. Don't make it any stronger than that. Too strong and it can cause just as many problems as it helps. Use a measuring spoon, not an eating spoon.
The damaged leaves won't heal. Watch the other growth and see if the discoloration, yellowing or browning in the middle of the leaf stops.
Nitrogen burns are generally at the tips first and then the outside of the leaf. Your problem seems to be discoloration in the interior part of the leaf.
Let us know how it works.