After editing a question's tags, its revisions page indicates that the r tag was both removed and added by me. Why wasn't it just left untouched?
This seems to be a side-effect of how the revision list calculates changes to tags, combined with how tags are sorted...
First the easy one: the tag list is compared in-place - so if a tag is moved from position #1 to position #4, it'll show up in the diff as being removed and then added.
Now comes the twist: tags are automatically ordered by frequency of use, but this is done only when a new revision is saved. So when your first revision was created on Stack Overflow, where r outranks latex,
r came first. When the tags were edited again after the migration, they were re-ordered to reflect their frequency here on AskUbuntu - where r is much, much less common. So it "moved".