Re: Drywall separates at wall/ceiling junction
Howdy sure sounds like truss deflection cracking. !/4" pretty big crack... Are you in a snow area- loading on the roof in the winter? Expansive soils? Before adding trim might just add a good quality white silicone/ latex caulk into the separation wipe off any excess. Caulking the crack to allow for movement. Caulk expands and contracts with the movement. Drywall tape and mud does not.
Any an all of my comments are just my opinion and not to be confused with facts.