We just tested this in the Tavern not long ago. These "no changes were made" edits occur when a user retags the question with a tag that gets remapped to a tag that was already on the question. The system accepts the edit as a new suggestion, even though the tag change results in the same set of tags that already existed, and thus the Diff doesn't see that anything has actually changed when displaying the suggestion.
I would classify this as a bug, because the suggested edit is essentially useless. No real change was made, it's confusing to reviewers, and most of all a complete waste of everyone's time. I've noticed these popping up more here on Meta from anonymous users trying to "comment" on the question not realizing that it won't ever show up anywhere. I really think that the suggested edit shouldn't be created until after it checks what the tags will actually be once remapped, and comparing against the existing tags.