I can understand wanting to use the full screen gestures on your device. When the navigation bar is set to gestures, you will need to swipe up on the gesture options to use them. You can change the navigation bar to the buttons if you would like by following these steps.
Go to Settings>Display>Navigation Bar
Once there you can select the Navigation Bar option you would prefer.
If you are experiencing the symptoms still, I would recommend testing the device in safe mode to see if the symptoms persist. You can do so by following these steps.
Press and hold the power button until power off, restart, and emergency mode show on screen
Tap and hold on the power off option until it changes to safe mode, then tap on safe mode.
The phone will then reboot into safe mode. You will be able to check that it is by a grey transparent box in the bottom left of the screen that says safe mode.
If the symptoms stop, this will let us know that there is a 3rd party app that is causing the symptoms. You will need to uninstall the most recent app since the symptoms started until the symptoms stop. If it continues to happen, I would recommend wiping the cache partition of the device following these steps.
Turn off the device.
Press and hold the Volume Up key and the Bixby key, then press and hold the Power key.
When the Android logo displays, release all three keys.
An 'Installing system update' message will show for 30 - 60 seconds before the Android system recovery menu options appear.
Press the Volume down key several times to highlight wipe cache partition.
Press Power key to select.
Press the Volume down key to highlight yes, them and press the Power key to select.
When the wipe cache partition is complete, Reboot system now is highlighted.
Press the Power key to restart the device.
Let me know if this helped!