I spent some time exploring tags today, and came across a few UX issues.
Firstly, are tag groups designed so that each tag can be in multiple tag groups? If not, could it be an admin setting to enforce this?
Then if so, it would help if the filter used when adding tags to a tag group excluded any tags that are already in a tag group. Right now it appears to only exclude tags used for the current tag group you are managing.
It would be so nice to have UI for assigning a tag to a tag group from the tag edit page. Right now you can only rename the tag but it would be helpful to be able to change the tag group from there.
I had multiple browser tabs open as a result, which led to another issue. I was adding tags to a tag group, and then realized I wanted to rename a tag that I had had already added. I renamed it in another tab. The old tag name was still listed as part of the tag group and I was able to add the new one.
And finally, this is still happening: