Not sure how to reproduce, It might be happening on map load sometimes, but not always.
You switch to melee weapon and staff is also partially on screen, moving together with melee weapon attacks when you perform them.
It has happened with Conflag and Fireball staff, several times, may be affecting all staves.
The only way to get it back to normal is to exit to desktop and start game again. If it happens in the lobby, getting into a new game does not seem to remove it.
I will record if it should happen again.
For now I turned off third person weapons mod and will see if that’s the culprit. Thanks @Perteks and if it’s about the mod, sorry for wasting your time, @FatsharkJulia :s
Looks like it’s from the mod. It has not happened again since, whereas before it would happen at least twice during a longer play session. I’ve added a comment to the mod’s author and disabled it for now. Thanks for the help!