Sure but for a moment let’s assume that a post was intentionally created in a category which had solved enabled, and then had an answer provided which was rightfully marked as the solution.
Would it make any sense to strip the solved status when a post is accidentally moved to a category where solved is disabled?
Remember, the category setting isn’t called “allow posts to show a solved status”, it’s:
So it’s working as advertised. Categories where that setting isn’t enabled don’t have the option to mark a reply as a solution, which is precisely what you experienced.