Vazkyanswered grow question 5 years ago I'm having the same problem now.. From what I've gathered:
It could be calcium deficiency, it didn't stop spreading after adding calmag so I ruled it out.
Water droplets on leaves burning them, I didn't spray them and it didn't stop spreading.
Phosphorus deficiency: most probably the cause, I have no way to add phosphorus at the moment because I'm on tight budget. The more I learn about phosphorus the more it seems like this. The plants need a lot of phosphorus in flower, which is when this started for me and seems like it started for you as well early in flower. So try adding phosphorus and see what happens.