I tried a few different combinations of this:
sharepoint.com/sites/pwa/_api/ProjectServer/Stages/getByGuid('uid')
Was expecting to get the stage value instead of just $true. I am misunderstanding the method documentation?
I also attempted your example above:
_api/ProjectServer/Projects('ProjectGUID')/Draft
But I get a 400 error.
If I remove Draft I get full output but I see return data only under Tasks