It's a mystery, but I'd run the chain for a stiff link just in case (my first rule of complex diagnosis, eliminate the easiest/cheapest first).

Backpedal slowly while holding the idler forward a bit so the lower loop sags. Watch and feel for any link(s) that don't unspool smoothly and straighten out immediately as they exit. Go a few times around to be sure.

Next pull down the loop and release a few times and see if the RD cage snaps back cleanly every time.