The steps below worked for my Samsung Galaxy Note 9 (T-Mobile)
Enable Zwift app for full screen: Settings - Display - Full Screen apps - Zwift . This should already be default, but check
Enable Full screen gestures: Settings - Display - Navigation Bar - Full Screen Gestures
Disable Gesture Hints: Settings - Display - Navigation Bar - Full Screen Gestures. Turn this OFF !
Step 3 was the key.
My reference was this reddit post; if the above steps don’t work for you, perhaps these will.
There are many devices and OS variations, so if y’all have other steps for your device please share!