It sounds like you have tinkered with the obvious issues; nock locator height and arrow spine.
Have you shot with broadheads and field points? If so do both shoot in the same group with no planing? If so, it doesn't sound like this is something that will impact your shot? Of course it is irritating. A lot of folks, if their field points and broadheads grouped together wouldn't look any further.
I'm wondering two things related to form.
1. Are you shooting split finger or 3-under? Have you tried the other way to see if the problem persists? Of course it shouldn't but maybe the string is coming off your fingers in and odd, vertical torquing way?
2. Your bow hand set. Does your bow hand wrist make an "L" with the life-line of the hand seated flat against the grip? With the pressure evenly distributed this will prevent heeling the bottom of the grip with the bowhand.
Kawika, good question about the double nock locators. That arrow could be moving down upon release.
Just fishing here.
Oh, welcome as a poster!