Defiancy or excess of something??

looks like soil too wet, maybe ph lockout due to that, or too much NPK so a Mag deficiency shows... how often do you water? do you let them dry so the pot is light? too wet soil makes ph fuckup ... too much nutes result in the plants being unable to suck out the water from the soil anymore, if your pot is rooted all through, and it takes long to dry with enough light to much nutes may be the problem. if that is the case, try flushing them with RO water or normal water without nutes, only cal mag normal dose if you use it, and some drain, at this size I would probably give a good flush.. not more than one time the pot volume. then let them dry through. really dry.

Its a deficiency caused by nutrient lock out. Feed is either a bit strong for the strain at the levels or feeding too often. The curling of the other leaves is also a sign. Temperature may also be a touch high. Adjust the nutrient level to a lower strength