fetchObjectFromURL with parent relationship fails validation

[jpaas]

I recently upgraded from 1.3.7 to 1.6.3 and I'm now having a problem when using fetchObjectFromURL. I'm fetching an order that belongs to a store location. The referenced store location already exists in the core data database prior to the fetch. Everything works fine up to the point of saving the backgroundContext. The fetched order is fully populated and the existing store location object has been correctly assigned to the storeLocation relationship attribute. But the save fails because of a validation failure on storeLocation and the dumped order shows a value of nil for storeLocation. Any idea what's happening here?

Just to confirm this is some sort of regression I reverted to 1.3.7 and it works fine.