Now we take it for granted, and Mac apps feel as good or bad as everywhere else.
The person who made a Qt app may be designing it based off of cross platform UX but maybe you should follow your own advice and not conflate things.
You can make a native UI and UX with Qt. You can also make things feel very non native via AppKit too. Your argument has very little to do with the toolkit and more to do with the developers choices.
I haven't seen what you describe, even if it's theoretically possible. I'd really love to be shown an example.
I'm not saying it can't happen, and in fact, if it existed I wouldn't have noticed (I mean, the point is that you think it's native, so I wouldn't have noticed..) but I can't think of an example and I would love to see one.
Don't confuse UI and UX. You can make Qt have the right UI, you can't make it have the right UX.