@gunther Thanks for the feeback, glad we could resolve the issue. Thanks again for reporting.

@gunther Thanks for reporting and sorry for the troubles!

The problem was with assigning the MaterialPropertyBlock at the part renderers when the Copy Property Block option is enabled at the SkeletonRenderSeparator component. If no MaterialPropertyBlock is used at the main renderer, it seems that uninitialized data is received in Unity 2023.1 and thus assigned at the other renderers. This issue has just been fixed.

New spine-unity 4.1 and 4.2-beta unitypackages are available for download:
https://esotericsoftware.com/spine-unity-download

This issue ticket has been tracked under this ticket URL:
EsotericSoftware/spine-runtimes2358
Please let us know if this fixes the issue on your end as well. Thanks for reporting!