Update: I took it off the camera this morning, left the rear cap off and played around with moving the zoom in and out a bit really slowly to see what happens. Here's what I noticed-
At 100mm, the contacts look like this:
https://flic.kr/p/2jYpuTT
However, if you extend the zoom barrel at all, the last one "recedes" and looks like this:
https://flic.kr/p/2jYqpTm
I'm assuming something to do with AF gearing is knocked out of place causing this. I don't have the right tools to dig in there and see otherwise I would take the mount off and try to DIY a fix. I don't know if paying for a fix would cost more than it's worth, tbh, but maybe it's something really simple after all.