I’d almost bet money it’s the otterbox. If it’s one of their older models, I would bet a small amount that’s what it is if it could be an anonymous bet and a good way to confirm. I’ve had a few otterbox cases over the years, and the older ones always degrade and get sticky, and it transfers. Their newer ones and colored ones don’t (or haven’t yet anyway), but I’ve stopped buying them because of it.
You can’t fix it, you just have to toss the damn things.
Dj Spiderhole
Aka DJ Sad Ahm