I think I've found the problem. It's different from the problem other people had before since this only happens at runtime.
There were some relatively recent changes to SkeletonRenderer that made some assumptions that weren't reliable.
Short-term fixes for you:
Either add one vertex to your Mesh.
or
replace this line in SkeletonRenderer.cs
: https://github.com/EsotericSoftware/spine-runtimes/blob/master/spine-unity/Assets/spine-unity/SkeletonRenderer.cs#L270
with
bool mustUpdateMeshStructure = true; //CheckIfMustUpdateMeshStructure(attachmentsTriangleCountTemp, attachmentsFlipStateTemp, addSubmeshArgumentsTemp);
The basic problem is that the renderer is optimizing updates by not refreshing the mesh structure when it doesn't detect "changes". Your current setup : a slot switching between a 4-vertex region and a 4-vertex mesh, (with the same number of vertices but different triangle definitions) fools it into thinking those two attachments are the same thing.
Fix #1 will make it so that it will detect the change in vertices. Fix #2 will force it to update the mesh structure every frame (this will fix it reliably, but has a performance cost). I recommend Fix #1. But use Fix#2 if you can't be bothered to update/re export too many Spine projects at the moment.