Just came back from a ride, what could this be? Shimano xt-derailleur btw

It’s jumping on your cassette. Derailleur might be out of adjustment – it looks like it’s trying to shift up slightly. And as the other person said, give it a cleaning. That may fix it. Paying attention to the chain and jockey wheels

Something similar happened to me recently. In my case, the 11T cog actually cracked and it was expanding when tension was on the chain.

Check out this post:

https://www.reddit.com/r/bikewrench/s/ILCo5dOrN5