I have experienced a wired scenario with my Apple TV 4K recently. When I connect it with a 1080P monitor, the default resolution is 720P while airplaying from my MacBook pro with M1. There is no other resolution options under the Scaled setting:
After some research, I found more resolution options are hidden under the Scaled setting. All you need to do is to hold “option” key and click the “Scaled” setting on the monitor with Apple TV attached. Now we can see these hidden resolutions:
Click the 1920*1080 one, and your are all set.