This is exclusive to the Unity 6 open beta branch, this does not appear in the public build.
Since first booting up the Unity 6 branch my main avatar has been stuck on a year+ old version and I just can't get it to update or switch from this ancient version. I tried clearing cache (all of it), reinstalling vrchat, uploading a new version of the avatar, switching worlds. Nothing breaks my avatar out of it.
I've tested a few avatars and my main avatar is the only one affected by this, the only thing I could point to as being unique or different about my main avatar is that it has over 300+ versions throughout the last couple years. I was able to skirt this issue by generating a new blueprint id for it and reuploading, that build of the avatar seems to be working and reflecting the current version.