If you update a relationship type for one person, you have to manually update the relationship type for the other person as well.
Slate doesn't automatically adjust because the relationship types aren't tied together such that one relationship (e.g. Parent/Guardian) is always and forever tied with its inverse relationship (e.g. Child). This allows for flexibility, but also requires more manual work. There's an enhancement request out there asking for Slate to make this kind of adjustment less manual: https://community.technolutions.net/advancement/post/updating-both-sides-of-a-relationship-AufEyyr7jLakVmY