It’s impossible to change the category of a topic to “Uncategorized” through the edit topic title feature. Changing to other categories seems to work fine.
Using the bulk selection tool to change the category to “Uncategorized” is working, so it seems isolated to the edit topic title feature.
Quick edit seems to be the only place having issues. Editing the first post and changing the category to “Uncategorized” through the composer works no problem.