This seems like a bug indeed.
A workaround that worked for me was to disable and re-enable Enable Render Separator
at the SkeletonRenderSeparator component:
I just filed an issue for it where you can track progress:
https://github.com/EsotericSoftware/spine-runtimes/issues/1269
The issue is now resolved. Thanks again for the bugreport!
You can download the latest unitypackage here as usual:
Spine Unity Download