It looks like when the animation to move the hotbar selector starts, the selector teleports back for some reason. This looks very weird and unsatifying. This issue is exagerated when changing to a slot far from the one you're selecting with the number keys.

This might be cause by the ressoucepack I'm using but it should be fixed anyways. It's also using the same size htobar selector so I don't know why it would do that.

Here is a vid of the issue in action:

Using ImmersiveUI v0.2.2
Fabric 16.5
Minecraft 1.21.1

Other mods and ressourcepacks used:

pastebin