You know a helluva lot more about shooting than I do but I'm not 100% sure about that. When I first started shooting again after an injury left me applying very little support hand pressure unless I really focused on applying it I was consistently shooting 3"-5" groups at 25 yards that were always high left. Not extremely high left, but consistent. It really felt to me like I was healing the gun at the moment I broke the shot. If I made myself grip harder with my support hand, which I could only muster for one shot at a time, and back off the strong hand pressure the high left problem went away.
As I healed up and my strength returned I would start training days shooting fine, but as my left arm fatigued I would catch myself compensating by using too much strong hand and pushing groups high left again.