Demonichka Thank you for reporting the problem with detailed instructions. I think this was a very difficult procedure to discover. It is a great help.
I have created an issue ticket to fix this issue here: EsotericSoftware/spine-editor807
We will fix it as soon as possible.

Demonichka Thank you for reporting this issue! I have confirmed that I can reproduce it, so I have created an issue ticket here to fix it: EsotericSoftware/spine-editor806
We apologize for the inconvenience, but please wait until we have resolved this issue.

We've fixed this in 4.2.24. Sorry for the trouble! It will be available soon.

Hi, I was able to reproduce the bug!
I've deleted Spinebot's answer cause it was not correct.
While we wait for the bug to be fixed, here's a solution I tested that works:
Export a json that includes nonesssential data. Import it and choose existing skeleton, and the skeleton you want to target. The slots should be named correctly!