[Xiaomi Mi 11] Asnamus:
I wrote email to Nova and I got instant reply:

"Hello,

Thank you for contacting Nova Launcher support. Below is a lot of information that we encourage you to read completely as it will answer your questions or other questions you might have related to the full system gestures in Android 10, Android 11 and going forward.

PLEASE READ THIS EMAIL IN FULL!

Home/Default Launcher Constantly Being Reset

If you are experiencing issues with Nova not staying as the default launcher on Android 10 or Android 11, these first steps are for you specifically.

If you are having issues with Nova staying as your default launcher, try this: Reboot your device. Or you can try Phone Settings -> Apps -> Default Apps -> Home App -> Nova Launcher. You might need to do this several times and reboot several times for this to fix the issue.

This is a very well known issue on Samsung and other devices, but specifically with new updates directly from Samsung starting as early as December 1st, 2020.

If this doesn't fix your issue, then you need to contact Samsung directly as the issue is on their end, with their update and has nothing to do with Nova or any other launcher, as this issue will happen in all 3rd party launchers.

The issue above isn't just for Samsung users, though the issue happens on every Samsung device that's gotten an update since December 1st, 2020.

Everything mentioned below is still present with the release of Android 11 on September 8th, 2020. Google has made it very clear that they have no plans to completely fix gesture-related issues any time in the near or even distant future.

Please take note that Google admits to this being broken on their end of things and that a fix will be coming in a future update to Android (stated March 2019).

However, it's now well over a year later and there are still many issues with full system gestures and 3rd party launchers on both Android 10 and Android 11. These issues will remain for the foreseeable future as Google have now made it clear that they have no plans to completely fix their full system gestures.

Notable issues with gestures on third party launchers include, but not limited to:

Close all/Clear app button not appearing(scroll all the way to verify it's not there)

Not being able to tap icons for a short time after returning to the home screen

Animations not being as fluid as expected

Screen blinks

Home screen icons blink

Home screen refreshes

Please understand, there is absolutely NOTHING WE CAN DO TO FIX OR CHANGE THIS IN ANY WAY! Contact Google or your device OEM directly with these issues as we have absolutely no control over it whatsoever.

Samsung Users

Samsung has allowed full system gestures to be used within 3rd party launchers with their One UI 2.5 update. This means you can now enable and use gestures, but there are still several issues regarding animations and the like.

As stated, there is nothing that Nova (or indeed any other 3rd party launcher) can do about this and it is ultimately up to Google and OEMs to fix navigation gestures.

Google's statement and Nova's statement

Google: https://android-developers.googleblog.com/2019/07/android-q-beta-5-update.html

Nova: https://help.teslacoilapps.com/faq/gesture-navigation

Enabling Gestures

As of the December 2019 Security Patch from Google, you can now enable full system gestures within 3rd party launchers on Pixel devices. However, the caveat of this is that you are likely to experience the issues mentioned above. To reiterate, this is still all up to Google to fix at some point in the future and to get things working 100% the way everyone expects them to be.

If and when you experience issues with gestures on 3rd party launchers, we highly recommend you contact Google and not Nova Support because, as we have said, there is absolutely nothing we can do to fix Google's problems.

Devices From Other Manufacturers

As some members in this Discord have noticed, some devices from certain manufacturers provide a reasonably usable experience with gestures. In particular, OnePlus devices on Oxygen OS 10 and above behave reasonably well with gestures.

This is because OnePlus (and other manufacturers) are using their own implementation of gestures rather than Google's which means that you might not get all of the issues referenced above. However, just like before, we (Nova) have no control over these and any issues you do experience should be brought up with the manufacturer and not us.

Action Launcher's False Claims

Action Launcher made an announcement back in late December 2019 which is completely misleading. When stating that they support system gestures, They are referring to the back swipe edit zones and other features within the app not interfering with Android's own system gestures.

As stated before, full system gestures are completely controlled by Android and are not something a launcher can just suddenly add support for or change in any way. When using gestures with Action Launcher, you'll quickly notice that the experience has just as many issues as it does with Nova or any other 3rd party launcher, reiterating the point that it is not something that any third party launcher can change or improve."

If I understood this correctly its MIUI problem...