I'm not sure if I'm doing anything wrong but I've noticed that any RelatedEntityList are not being copied correctly into a sandbox entity manager.
Lets say I have a customer that has a contacts collection. I use ImportEntities on the customer object to import it into a sandboxed entity manager. In the sandbox the contacts collection (which is a relatedEntityList) becomes a 0 length collection with IsPendingentitylist=false. This happens when contacts collection is loaded (not pending) or not.
This is the sort of thing I'm doing. Where myEntities is the entity manager and myCustomer is the customer from the model. mgr is the main entity manger of type myEntities.
var sandboxEM = new myEntities ();
sandboxEM.LinkForAuthentication(mgr);
sandboxEM.ImportEntities(new object[] { myCustomer}, MergeStrategy.OverwriteChanges);