Add / Remove Tag using checkbox

The button isn't the same. In the first one you choose the tag to add, in the second, you choose the tag to remove. I'm not sure what the issue is?

There are 2 options: add / remove

When you click on one or the other, the result is the same: a box inviting you to "add a tag" and when you click you get the list of all the tags that you can add. You can even select a tag that is already present. And to delete, we therefore have the same box described previously and we can select tags which are not present and therefore not to be deleted.

Well, you can select multiple topics there, so we're not going to look at the current tags on those topics because they likely won't be the same. If we would only allow you to add a tag that wasn't present in any of the topics (for example), then that would become very annoying and would defeat the purpose of allowing multi-selection.

Same logic with removal.

This is working as intended.