My index finger involuntarily wants to straighten away from the strings. Mostly when the thumb drops down to other strings. Anybody fight thru that?
Naturally, I know nothing. But it sounds like either
a) bad old muscle memory
b) index finger is getting ready to pick next note.
c) some old, unknown football injury that’s just now making itself known.
Solution? Probably the standard answer: Practice.
It could be an issue with your picking hand’s position; if you upload a video here of your picking hand, it might help others identify the cause.
Probably just a bad habit. Practice a lot of rolls and be mindful of it and force yourself to keep the finger close to the strings.