I don’t have an answer to your mystery, but I have seen the same thing.
I have had some strings that just keep creeping. Don’t know why. Replace the string, everything is fine.
I have had some bows that relax a bit after being strung. I would check the brace height when stringing the bow, and it would be up to 1/4 inch higher than what it would be after shooting the bow 10 times. The measurements were repeatable, so I just didn’t worry about it. Seems to me most bows do this at least a little bit.
But none of it has really been enough to mess up arrow flight. Maybe something needs to be tweaked on your bow to make it more forgiving. Like move the string nock set up a bit? Or increase the brace height a bit?
Or maybe the bow weight is too much. I find that if I am shooting a heavy bow, things go well for a while, but then fall apart as I get fatigued.