wpExistingTags is not passed in the request at Special:EditTags if multiple revisions are selected.
It follows that the option to remove all tags cannot work.
Also, users are prompted to add/remove tags that they cannot add/remove.
I think we should check beforehand and only propose them tags they can add/remove.
I had done this in commit 201905 but will split it off since it's not related to the main issue of that commit.