When Android started using rounded corners, it was the devices that first did it, with apps following suit later on. I still have a few legacy apps that are squared in the corners but on my rounded edge display, some of the actual content is obscured by the rounded edge.
Here, it's the opposite - Microsoft has implemented the rounded corner in the OS first, but my display still has square corners. naturally there are going to be applications that are not coded to use rounded corners, as the OS was just released to Insiders 2 days ago and the previous leak is barely 2 weeks old.
I suspect more and more developers (and even Microsoft themselves) will slowly but sure start converting their apps to rounded corners, as time progresses.
Luckily, full screen and maximized apps mostly work perfectly fine with squared corners still.