Title pretty much explains it - .sublime-project files open as:

{
	"folders":
	
		{
			"path": "project-name"
		}
	]
}

And .sublime-workspace files open similarly, however if I open a .sublime-project and then a workspace, I get the behaviour I expected on opening a project, i.e. all my previously open tabs / window settings etc are restored.

Yes, that’s how it works. To open a project file as a project, use Project->Open Project from the main menu.