This seems like a legitimate bug to me. I’m going to delete our previous replies to make the bug report easier to read. If this is not a bug, it is at the very least a UX issue.
I’m testing this with a category that requires at least one tag per topic:
On my site, the
min trust to create tag setting is “1:basic user.” I have tested this with both a tl1 and a tl3 user. These users are able to create new tags on the site.
When creating a new post in the category, the user attempts to create a new tag:
Clicking the “Create Topic” button returns the “You must select at least 1 tag” error modal. If the user then keeps the
foo-ba-rbaz tag in place, but also selects an existing tag from the tags list, they are then able to create the topic:
The user is obviously also able to create a topic just by selecting one or more existing tags.
The way this is working does not seem to match the category setting description. That setting just asks for the minimum number of tags required for the category’s topic. It seems strange to require one tag that already exists to be added to the topic for users who have permission to create new tags on the site. Requiring an existing tag to be added to a topic would be better handled by the Tag Group setting that is found in the Tags section of the category edit pages.