DennisPI tried different combinations, like turning the one-handed mode off, turning on the navigation bar, turning on the one-handed mode and going back to gesture navigation, repeated it in all the different orders, but nothing worked. The moment I turn on gesture navigation, the gesture for one-handed mode stops working. It used to work without any problems, but a handful of system updates back it just stopped.
I use gesture navigation since day one and never had a problem with one-handed mode.
Have you tried to check/adjust settings again for one-handed mode after enabling gesture navigation?