We tried removing a tag while in edit mode by scrubbing it from the tag field and clicking on the list of suggested tags
I just added a thread with two tags, edited the thread and deleted the second tag and saved and it worked fine. When you say that you clicked on the list of suggested tags, what does that do? The only thing you should need to do would be to remove the tag that you don't want from the tags textbox.
This problem happens only when deleting that last tag for a discussion. If there are more than one tags and if you try to delete every tag except one, that works fine. BTW I tested it in 2.5.5 Beta and it works fine in that version.