When I play APOTU on my Index, the game seems to think my right thumbstick is always pointing diagonally up+left, even when it's physically not. (The model shows the stick pointing up+left, and I seem to be permanently in 'teleport destination select' mode, with no ability to complete the teleport.) The left thumbstick and both triggers seem to work correctly, and I'm able to grab things.

SteamVR is able to see the real state of the right thumbstick, so I don't think it's a hardware issue. I tried closing and relaunching APOTU, closing and relaunching SteamVR, and turning my controllers off and back on, but the problem still happens every time.

Any ideas what might be wrong?

Ouch, sorry to hear! Can you check the mapping you use for APOTU (in the steamvr controller settings)? It should be set for default, maybe theres a rogue custom mapping active?

You could try to also callibrate the thumbsticks (also in the steamvr settings).

I'm running it on Index myself, so it should work just fine. However, APOTU uses an old version of Unity's Input system, which isn't the greatest in combination with Index, I needed to apply some trickery to get it to work decently.