I had the same problem and decided to dive in the source code to look what’s causing this. I found some wp_unique_term_slug function which mentions in a comment:
“The $slug has to be unique global to every taxonomy, meaning that one taxonomy term can’t have a matching slug with another taxonomy term. Each slug has to be globally unique for every taxonomy.”
So apparently you can’t have categories and tags with the same slugs, so I deleted the tags with the same name and after that I was able to make the categories without the slugs getting renamed.
Still kinda silly that it works like this though.
