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Which ROM version are you on? Have you turned on the toggle?
Just checked One handed mode with navigation buttons on 10.3.7.0 and it works fine. But to enable it while using fullscreen gestures, you need to use 'Quick ball' feature.
Check out this thread to know how to enable One handed mode using Quick ball :
One Hand mode in Full Screen Display mode https://c.mi.com/thread-1774503-1-0.html
P.S. - You need to swipe more than half of the navigation bar to trigger this gesture while in button mode. For gestures, it should be easy with Quick ball.
If you still can't get it working, I would first suggest you to MAKE A BACKUP and do a factory reset and check this. If it still doesn't work, send a feedback with a video sample of your issue (taken using another device) so that the Feedback Team can figure out the issue.