The odd thing is that only some leaves in the middle of the plant are affected. My thoughts were Nitrogen or Manganese deficiency or maybe light burn (why in the mid of the plant tho?)? I feed Biobizz Bloom, Grow and TopMax, no Problems until now.
light stress is induced with either too much or not enoguh light wich ic common bottom of middle of canopy there's nothing especially wrong with the the plant slowly cannibalizing those leaves along flowering.