If I were to take a stab I would be looking at P&K as the causing factors here also.
It's pretty obvious that K's had an issue here at one time or another.
Likely a ratio thing as aqua suggest. I would consider restricting phos a little, and see if things improve. These things are always an educated guess, nothing more, IMO. But phos is certainly one of those that are commonly overdone IMO. Especially if you were pushing it early.
I generally have evolved into not even increasing it until almost the 2-3rd week into flower, and if it's not being up taken, it's simply accumulating in the substrate. Phos doesn't leach very well, that's one of the reasons I've started to restrict this mineral more. K generally will burn or whatnot, but it's seemingly easily flushed out, otherwise.
Phos is really a troublesome mineral on many fronts IMO, especially in soil or substrates, kills of your fungal biome, ect...
disclaimer: I'm not suggesting this is the problem 100%, but I've seen it cause problems such as these.