[Google Pixel 5 Questions & Answers] akulp:
I wrote to Nova and there is official answer:

Hello,

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. If that doesn't work, read below as it's been a known issue for well over a year now.

PLEASE READ THIS RESPONSE IN FULL!

The things mentioned below are 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 these issues any time in the near or even distant feature.

You can read Google's official statement and our statement about the issues you and anyone else using Android 10 and Android 11 are experiencing. 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 and Android 11. These issues will remain for the foreseeable future as Google has no plans to completely fix their full system gestures.

This includes things such as the CLOSE ALL/CLEAR ALL button not showing up when you access the Recent Apps screen. As well as a delay of being able to tap on app icons when returning to the home screen from another app, from viewing the Recent Apps screen or anything similar.

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 contacting any launcher developer isn't the correct thing to do and isn't going to accomplish anything.

SAMSUNG USERS: Samsung has enabled full system gestures to be enabled within 3rd party launchers with their One UI 2.5 update. This means you can enable those gestures, but things are still very broken, especially when it comes to animations and how smooth the gestures work. This is NOT something Nova, or any other 3rd party launcher can fix. Only Google can fix it.

In the end, none of this is something we(Nova) can fix or change in any way and neither can any other 3rd party launcher for that matter. It is ultimately up to Google and only Google to fix full system gestures completely. Though they have made it clear that full system gestures will remain broken even with the final release of Android 11 and likely well beyond that.

We understand that this statement was made during a beta version of Android 10. However, as is stated in the below URL, this issue will remain broken even after Android 10, and now Android 11, is released in its final form. What this means is that Google knew well ahead of time that they had no plans to fix this until much later so it's still a relative issue today.

GOOGLE'S STATEMENT:

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

OUR STATEMENT:

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

As of the December 2019 Security Patch from Google, you can now enable full system gestures within 3rd party launchers. However, please note, there are still a lot of issues with the gesture system such as animations not working, Close All button not showing up and several other things. This is not something any launcher developer, including us here at Nova, can fix or change in any way.

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 you experience any issues at all while using full system gestures, it is highly suggested that you contact Google directly and/or file bug reports with them directly so that your voice can be heard by them directly.

ACTION LAUNCHER'S FALSE CLAIMS: Action Launcher made an announcement back in late December 2019, is actually misleading and 100% incorrect. They are referring to the back swipe edit zones and similar stuff. Full system gestures are completely controlled by Android and not something a launcher can just suddenly add support for or change in any way.

If you actually try to use the full system gestures in Action Launcher, you'll see they are still as broken as they have been and will remain that way until Google fixes things themselves.