FAQ: Why are my relationship class joins missing from my map document?
Why are my relationship class joins missing from my map document?
If you move a project and related data, ArcMap can not repair joins based on relationship classes, even if you save the document using the relative path option. You can easily recreate the join and save the project.
If the project and data is to be moved again, you may want to use attribute joins instead. These types of joins are automatically repaired by ArcMap.