If I categorize a previously uncategorised post, it puts that on top of the list of posts. I don’t think that only changing/assigning a category warrants a bump.
Yeah, this has been discussed a lot, at the moment if a topic has only 1 post a recategorization will bump. This is by design.
We are considering adding a feature of “non bumping” edits for moderators which will allow them to better categorize stuff.
Additionally we may consider adding a site setting that allows site admins to elect that “Category changes” and “Re tagging” is not bumping.