How many times are we supposed to repeat this? On Android 11, the full-screen gestures are provided by the MIUI System launcher, and there are no fallback gestures provided by the system, so you cannot use them if you set any other launcher as default.
If you had the MIUI launcher set as default, then you switch to another launcher, gestures will still work because the MIUI launcher is still running in the background, and will remain so until you force-close it (either by rebooting, clearing memory from Recents panel, or any other method), then the MIUI launcher that's running in the background will be terminated and full-screen gestures along with it.
Read the sticky before posting: