Right now, custom fields on posts and topics are saved at the time of posting for category experts. This means that posts in a category by an expert, before they were in an expert group, are not marked as expert posts. The default settings capture the moment in time when the user is posting.
This setting makes it so that when you make a user a category expert, all their historical posts get the custom fields saying it’s from an expert. Then to follow, then a category expert is removed as an expert, their posts lose the custom fields saying they were ever from an expert.