Confirmed the issue in 6.10.5, but just with the 'owner' of objects..
It appears that the System.owner is not emptied when deleting the account that created the object.
I'm not sure if that is a bug or intended behaviour. Mendix treats this somewhat differently: it is not stored in an association table, for example.
In any case, it's probably better not to expose that association, but instead define your own association between user (or account) and your entity (car).
Those associations are emptied correctly and do not give any problems in the OData service.
Nevertheless, you can file a ticket to see if this is the intended behavior for 'owner' references.
Hope this helps!