That seems like something that should not be allowed to happen. I’m assuming that you ended up with something like this:
I’m testing this on my local development site. After creating the synonym, clicking the “Remove Synonym” button gives a 422 error. Clicking the trash icon for the synonym doesn’t give me a 500 error, instead it completely deletes the tag.
I think some protection should be added to prevent a tag from being set as its own synonym.