Oh dear, sorry to hear that!
I have uploaded another update which improves the situation in a different way and fixes another potential problem (however I'm not sure it fixes yours):
Spine Unity Download
Please let me know if the issue still persists. If so, could you please share another screenshot, just to be sure the call stack is still the same and the problem still occurs at the same line of code. Could you then please also create a minimal Unity project that still shows this issue, and send it as a zip package to email@example.com (briefly mentioning this forum thread URL so that we know the context)?
Replying here on the forum to your email so that others can also benefit.
I'm sorry to hear that the problem still persists, thanks for reporting!
I found that if I comment these chunks entirely, we have no problems with build, and it looks like everythign is fine, but I don't actually know what this preprocess step does and if it is a problem that we are disabling it. Can you tell me what this code is for and what the effects of just commenting it out would be, and if it is essential, do you have any ideas on how we can fix this issue?
PreprocessSpriteAtlases are non-critical, it only reduces build size by removing unnecessary Mesh and Texture reference which is needed for the Unity Editor only, and not in the build.
The first method
PreprocessSpinePrefabMeshes clears the saved procedural Mesh at a SkeletonRenderer prefab. The procedural mesh is only saved at the prefab so that Unity does not display the prefab as "modified" all the time. Clearing the mesh should reduce the build size, removing this method should thus only have the effect of increasing the build size slightly by the mesh size.
The second method
PreprocessSpriteAtlases clears the generated Editor Sprite Atlas Texture at a
SpineSpriteAtlasAsset, which is rarely used anyway. This texture is again only generated and assigned at a Material for use in the Unity Editor, and not used in the build. Thus as above, removing this method will only increase the build size if you are using
SpineSpriteAtlasAsset (which I don't assume) bu the size of the generated editor atlas texture.
We will have a look at the issue again and will let you know once we have an update to offer which cleanly resolves this problem.