With some testing with the tagging I see the same:
Once a tag has been defined AND at least one file from the “general” media room has been assigned to this tag, new files can directly be added to this tag.
As you say, it does not appear to be possible to add media directly into a tag with no media in it.
Also any media added to a tag that is already in the media room either tagged or not, will not be added to the selected tag. Media can be tagged in more than one tag.
Maybe submit a suggestion to (reinstate or repair) the adding of media in empty tags?