@Fry Thanks for sending the reproduction package! I just noticed that the necessary bugfix that allows repacking of sequence attachments was only published to the 4.2-beta branch, since it contains risky changes that might break existing 4.1 projects.
See the issue ticket here:
To integrate the changes to your 4.1 project, you would need to apply the bugfix commit manually to your spine-unity runtime's
Since you're using
Add package from git URL to add the spine-unity package, you would need to instead download the package, modify it accordingly and add the package via
Add package from disk.
Sorry for the inconvenience!