@Sergey Thanks very much for the additional info! I indeed must have misunderstood this part of the Unity discussion thread, that it also affects OnPostprocessAllAssets.

It's a pity that effectively "Everything is allowed in this callback" seems to translate to "It doesn't explode, but some calls will no longer have the expected side-effects and you need to call experimental undocumented functions to write your assets to the artifacts directory, then hope it also gets imported and also written to the target Assets directory." This is not exactly great design and documentation on the Unity API front.

@Sergey We have pushed a bugfix for the Parallel Import issue to the 4.2-beta branch. A new spine-unity 4.2-beta unitypackage is available for download here as usual:
https://esotericsoftware.com/spine-unity-download#spine-unity-4.2-beta
Please let us know if this resolves the issues on your end as well. Thanks again for reporting!

@Sergey Hm, this is strange. According to this posting by a Unity developer from Nov 2023, OnPostprocessAllAssets should not pose any restrictions:

"The documentation is correct. The naming of "OnPostprocessAllAssets" is misleading in that it is not a normal postprocessor (those are being run as part of an import of a specific asset). It is rather a callback done after all assets have been imported. Everything is allowed in this callback."

Did you ever receive the warning you mentioned, "AssetDatabase.CreateAsset() was called as part of running an import."? Or did you just see it in code?

@Sergey Thanks for reporting. Does this also occur when you disable Parallel Import in Unity's Project Settings?

It might be related to the issue reported on your other thread:
https://esotericsoftware.com/forum/d/25894-the-apply-atlas-texture-settings-is-not-working

@Sergey Thanks for reporting and sorry for the troubles! This is indeed a bug. We've opened an issue ticket here:
EsotericSoftware/spine-runtimes2496
We will let you know here on the forum once this issue has been resolved.