If I had to pick a top 3 guesses on the issue/solution.... (#3 is my pick though ... well... maybe #1...)
1) Although your no3/po4 doesn’t seem to be in “danger zone”, I’m sure you’ve read a lot on low no3/po4 issues already on r2r and people have mentioned it here. But this one in particular seems a bit similar (his po4 was much lower though at .002) with signs of success from raising no3/po4:
https://www.reef2reef.com/threads/fading-tissue.643267/
So maybe blitzing the po4 up at least... dosing or reef roids
2) If you don’t want to raise nitrates/phosphates, maybe drop alk closer to 7 (although... In your email you said it was 7 and raised to 8 just from salinity correction? So is it usually at 7 dkh? And issues were still present? If so, obviously this is moot. And even 8 is not too bad but eh... long shot)
3) Swinging back to flow... still on my mind that the rack getting hit with whatever makes it to them along the glass from 90% power of a 6000gph rated gyre push being too much... i know it doesn’t apply exactly but if you imagine that zone as a 50g tank (since it’s in that 1/4 area).. do people think their sps will be happy with a 6000 gph rated gyre blasting at 90% from the back corner in their 50g every 3 min for 3 min in one direction? On top of whatever the other 3 pumps contribute (Not rhetorical, really asking the people haha).
If not the strength of flow, maybe just getting hit from one direction mostly...but that’s also just referring to that frag batch since I don’t know where past unsuccessful frags have been placed.