Anyone have a clue what this problem is?

Leaves start browning and dying off at the center. They get way worse than this but I removed the worst affected leaves before I decided to post the problem here. Just 3 or 4 plants have it in a 50 plant grow. Only seems to affect fan leaves so far. Some sort of root problem?

top or bottom leafs? new growth old growth

Fan leaves only so far - I'm 4 weeks in flower. It shouldn't be an iron deficiency - been feeding my plants cal/mag + iron for a few weeks. The problem appears at the same location but the leaves turn yellow/brown and start drying out pretty quick. Runoff is 6.5 so it shouldn't be a PH problem neither.