Relationship fields to not work like standard field. Relationship field store a serialized array of values. You are basically trying to create a 1 to 1 relationship but the relationship field is a 1 to many relationship.
First I would suggest switching to a post object field, which by default is a 1:1 relationship.
If you can’t do that then I’d suggest reading this https://www.advancedcustomfields.com/resources/querying-relationship-fields/
