Check your growing medium's ph, could be ph swings. I have the exact same thing happened using Massive but not sure that is what is causing it. My coco/soil's ph drifts up overtime especially later in flower when the plant is feeding less. My soil pH is around 6.4-6.5 and feed water anywhere between 5.5-5.8, they don't like it, I usually run a small flush with nute solution about half strength, 5.5 ph until I see soil runoff drop around 5.8-6.0 then lower my nute feed to 500ish ppm until the end.
I can personally rule out heat/leaf temps and light stress. My leaf temps are always 24-25° and my lights are 28inches away at about 70%..(about 400watts). RH 40-45%
Massive is not for finishing, don't use it past end of week 5 or your buds will keep throwing white pistils and foxtails..guaranteed. No need to talk to regional manager about that...
Hope it helps wolf