see https://raspberrypi.stackexchange.com/questions/100312/raspberry-4-usbmount-not-working

I faced the same issue with Raspberry Pi 4 and Raspbian Buster, 
the solution for me was to modify the following file:

   /lib/systemd/system/systemd-udevd.service

Change: PrivateMounts=yes

To: PrivateMounts=no

Then I rebooted and it was OK ;)