A category has a defined allowed tag group.
When you create a new topic within this category and create a new tag, this tag is unassigned to any group. This is a contradiction, because an unassigned tag isn’t allowed in our category.
Shouldn’t a new tag be assigned to an allowed group by default?
I fixed the two issues you found. You can create new tags in the tag groups (as shown here) and you can’t create new tags in a category that only allows tags from a group.