I think there’s a need for real world use cases similar to yours to help improve the tagging UI.
This is probably because you are testing this as a staff user. Tag restrictions aren’t applied to admins (possibly not to mods either) so what you are seeing doesn’t reflect what a normal user would see.
Here’s my test with a normal TL3 user (note that it’s only allowing me to select from products at first):
Here’s what I see when I test the same thing as an admin user (tags from both tag groups, returned in alphabetical order):
This difference can lead to confusion when trying to configure tag group permissions.
Yeah, that’s a problem.