Using the latest version of Anki Desktop, I have trouble getting tags to save when I change them using the Edit button or when creating new notes. If I change tags while browsing through my cards, I have no difficulties.
Did you rule out add-ons?
I don’t use any addons so it can’t be that.
Tag editing has been changed up in 2.1.50, so chances are this has been resolved already. Please try the beta and let us know how you go: https://betas.ankiweb.net/
For more details, it seems that this happens when I have my cursor in the tags box while hitting ctrl+enter to save my changes.
Yes, I also actually noticed this issue a while ago. It happens when you click Ctrl+Enter after selecting a tag from the autocomplete list using the keyboard without closing the list first. Not sure if the new tag selector has this issue too.