MyApex Legends is fullscreen my mouse leaves and goes to second monitor

Hey there, 

If disabling the overlays didn't work, you could perhaps try a Clean Boot in case there are any other apps that might be causing this: How to clean or safe boot your computer

In case the problem persists, this player said in this post that they managed to fix that by using third-party software to lock the mouse cursor to one screen only while playing. Please note that we do not provide support for that software, so use it at your own risk.