I'm currently not using magisk.I fix it like this if you have magisk install
Install the zip in attachments Navigation_Bar-Android-Pie-OnePlus-3_3T.zip. then go to settings/personalstion/button you will see below hardware keys disable keys press it then reboot now when you phone reboot it will boot with what ever you choose under settings/System/ gesture.
!Note if you reboot before disable keys you will have both nav bar buttons and hardware buttons working on the same time. And you can't disable hardware keys then because you don't have the option except if you uninstall the zip .
I tend to use the PS4 remote app a lot, and it doesn't work if magisk or no verity are installed.