I am glad you have figured it out though I am not sure I would label it user error so much as user input creating the situation. There are a variety of hand sizes and grip variations as well as obviously different ergonomics and control layouts on pistols. Sometimes that stuff just has to be figured out and adjusted for. Most likely your grip is changing slightly when you really bear down and shoot aggressively.I shot this morning watching my thumb. I really think that was the case. I suspect suspect it is user error. I’ll keep shooting and see if it appears anymore.