I don't know is it really bug or feature, but for me seems it definitly first.
When I give some of my sprites names like something_something.2_2 where the last 2 can be any other number, and export the project, in atlas this sprite losts x part of it's name and it's index become other, depends of the number (although it is usually -1). Cause of this, skeleton viewer doesn't opens this spine correctly and I can't use it properly in my project (unfortunately, I can't give you a specific example file because of the NDA).
My spine version is 3.8.84 Enterprise
It's not really comfortable fix each time all cases with it so hope you will make it work well. 🙂