Seems like my plants always get some kind of funk shorty after starting flower. No matter what kind of symptoms or deficiency it shows, it's always come down to my pH...
Doesn't matter which soil, I always end up too acidic. My well water is between 7.2 and 7.9 pH and even watering without adjustment my soil still gets down to the 5.5 range. Probably just the change in root exudates during flower causing mine.
Only thing I've found to work is mixing some garden lime into the soil, or top dressing and watering in. Also get extra cal mag from it so bonus!
That's organic though, if I'm using salts I just flush and make sure to water to runoff each time.