Week 6 flower update, ran into a weird lockout

Running a Zkittlez photo in my 4x4 right now, DWC with a 600w HPS plus a 200w Samsung QB for supplemental. She's been a heavy feeder this run. I had her dialed at 1350 ppm (Bluelab, so 0.5 scale) and EC sitting at 2.7 through week 4 and 5 with zero complaints.

Hit week 6 and I started seeing interveinal chlorosis on mid-canopy leaves, classic magnesium deficiency look. Thing is, i'm running full Calimagic at 5ml/gal on top of the base nutes, so mag shouldn't be the issue. pH was reading 5.9 at the reservoir, which is where I run it. I bumped pH down to 5.7 and did a 30% reservoir change, brought total ppm back to 1280.

Three days out from that change and new growth looks clean, but the affected leaves haven't recovered obviously. My working theory is that the high EC was causing a calcium/mag antagonism at the root zone even though pH looked fine on paper. Reservoir temp was also creeping up to 73F last week during a warm stretch here in Denver, so that could have played into root efficiency too.

I've got a chiller on order but in the meantime i dropped two frozen water bottles in the rez yesterday and got it back to 67F. Anyone dealt with a similar lockout situation at those EC levels mid-flower? Curious whether dropping the ppm further made sense or if I should have just pushed through with a flush.