When the player moves at high speed (locomotion) over about 2m/s physbones will flail around wildly. This would be acceptable except for that it also happens even if they have their Immobile set to 1. It doesn't make any difference if it is set to "All Motion" or "World".

This negatively affects avatars such as avtr_ba3daad5-0c64-4fb4-8593-38265acd9ad3 which use physbones to measure the up direction. When walking briskly the angle of the measurement physbones will flail around wildly causing undesired animations.

I feel like this wasn't originally an issue with physbones, and it started happening relatively recently, but I don't have any hard evidence to back that up.