Field data lost at data link synchronization

I'm trying to link a field to a data linked pipe network structure. When I insert the field and define it to the object name for the structure everything works great and the Manhole number shows up in the field. The problem comes when the pipe network is synchronized, either when opening the drawing or manually, the field loses the object. When I go to edit the field I get a "The object referenced by the field is not a valid object" message. Any ideas.