Should civicrm_post hook receive populated Relationship object following relationship enable/disable in UI?
Sorry but it's the intended behavior. Generally pre and post hooks just pass whatever happens to be loaded in that object rather than taking the performance hit of doing a full database load for the sake of a hook which may or may not be implemented and may or may not care.

I recommend not relying on the object being passed to pre/post hooks and doing your own api lookup using the provided id.