AirPlay turns my secondary monitor into a mirroring device

I just replaced a BenQ monitor by an LG Ultrawide one as a secondary device to my 2017 Macbook Pro. When using AirPlay to my Apple TV (AirPlaying the Macbook screen), I used to be able to keep using my secondary monitor as if nothing was happening. However, since I changed my secondary screen, turning on AirPlay automatically turns on this secondary screen into mirroring mode (with a terrible stretch due to the aspect ratio differences). Any idea how to fix that?