FastSpringBone is a feature that optimizes the calculation of swingable physics for avatars. It emulates the behavior of the original SpringBone, but it is much faster. However, there is no guarantee that FastSpringBone works with every model.
For detailed information on the new behavior, refer to this blog article (Japanese).
If disabling FastSpringBone solved the problem,
the cause could be that the “Pose Freeze is not executed when the VRM was exported” or the “VRMSpringBone is set incorrectly” as described in Model is not displayed correctly or the model is corrupt.
In the future, models with these issues will be denied during VRM export, so it is always good to fix them as early as possible.
If the model looks strange even after disabling FastSpringBone, there may be an issue with the model.
Please contact the creator of the model.