I have noticed that the when using "lock head" mode in the new IK beta that the spine of some avatars will twist erratically.

I was told that this is due to some avatars hip root bone not being oriented up correctly as the new IK in lock head mode does not account for non zero hip rotation.

While it is good practice to have the hip bone oriented properly (as the SDK has been warning us about for years) this is the first instance of it affecting avatars in a tangible way. This will break years worth of content if not accounted for, some of which reuploading or remaking is not feasible, there are tens of thousands of avatars which have an inverted pelvis already in vrchat. I know my current base that I sell suffers from this and I do plan to make a patch to address this issue regardless, but it will not fix the 2 years worth of content people using my base have already made, it would also break existing animations that use the tail. I know that the rexorium base suffers from this issue as well as other popular base models.

Another issue that this change doesn't account for is non blender users. Maya for example has a different way of handling bone orientation than blender does and if a model needs to pass through both modeling suites a hip error like this can occur even if its accounted for in one of the programs.

While it could be a slight performance or accuracy hit to check for hip orientation on avatar load I feel it is a worthy sacrifice in the name of preservation.