I double-checked, rebooted and checked again. The back button doesn't do anything since the latest update. Not using the navbar and not using gestures. It's on Android 8.1 on a OnePlus 6. I'm going to try and rig up a workaround using Tasker. If this problem is to obscure for an easy fix, could you add the option of using system keyboards? That way I could use the esc-key on hackers keyboard as a workaround.
Edit: when I bind a gesture from GMD gesture control to "back", it just exits the app.