too much heat
the burn on the leaf LOOKS a lot like too close to the light type burn, but you're in a greenhouse, so... too much heat
also, too little nitrogen - you've been feeding flowering nutes exclusively and are in week 2.5 - the plants stretch(veg) for the first 2 weeks of flower and burn up a lot of N creating new foliage - the flower nutes are low in N on top of this
prolly a mild cal deficiency as well
go ahead and flush the soil if you wanna just to be safe(won't hurt) but the next time you DO add nutes, use veg nutes and see if your plants don't green up overnight - LITERALLY, you will SEE the green PUSH OUT the leaves
get a bigger(or another) fan - lower that humidity or you're gonna be RAPED by mold
also, get some duct tape and cut some vents up high, if necessary to lower your heat - you MUST lower heat and humidity in there man
get those plants some nitrogen EVERY time the leaves fade
i feed veg nutes in veg and 50/50 my veg/flower nutes in flower, compensating where necessary
DO look for bugs, but thesed are all DEFINITELY issues as well