This is a new one. The topic tags are updated on each reply with the contents of the input box. This is by design but will only result in empty tags if you reply to a topic with the tags input empty, so that forum members can self-police them. They are replaced as easily as they are deleted.
One current drawback is if you trash or spam a topic, it could leave empty tags behind. I’d maybe open a ticket for it, and I can look into it further. Right now I can’t reproduce the bug, but that doesnt mean it isn’t one.
Upon replying to a topic, the tags input box was being pre-populated with existing tags. I thought I could go in and wipe them out (of the box), input my own “new” tags, and all tags would append to a topic.