My guess is cal mag def, caused by low ph. Or a lockout. I think under 6.4 it cant uptake calmag in soil iirc, you may want to check that number its been a while.
I used to get a few calmag deficiency due to low ph, i was getting the same runoff ph as you, as my tapwater ph level is all over the show.
I use dolomite lime to keep the ph in the perfect range, flush once after putting onto flower just as a preventative and havent had any nutrient deficiency since.
Maybe think about it for your next crop.
Probably to late now, maybe try ph up and get your feed to 7.5 check the runoff should be high enough for the plant to uptake the calmag. Unless its locked out but your flush should have got rid of the build up.
Yes lower humidity is better in later flower.
Good luck
