Overview:
Changes made to a Vault Document Relationship configuration are not applied to existing documents.
Root Cause:
This is working as designed.
Solution:
If a user applies a Document Relationship on a document prior to a Document Relationship configuration change, it does not adhere to the new configuration. The solution is to remove and re-add the related document in the affected Document Properties so that it can reflect the updated Document Relationship configuration.
Related Documentation:
Vault Relationship Documentation: How to Add & Remove Relationships