I would check the database to see how your adhoc relationship is being represented. It's the CMS_Relationship table.
In the table itself, there is a "RelationshipIsAdHoc" so it could be that by default this is filtered to where only Null or 0 (false) are selected, and if it's (1) it's filtered out. I don't see a property on the repeater to specify this so not sure if it can be tested.
Another option is to use a normal repeater, not use the related fields, and simply use the WHERE condition to formulate your query like the below:
Where: NodeID in (Select LeftNodeID from CMS_Relationship where RightNodeID = {% Convert.ToInt(CurrentDocument.NodeID, -1) |(identity)GlobalAdministrator%})