I just wanted to share an update on this topic that I found elsewhere:
The trick with setting `MountFlags=shared` does not work on Ubuntu 20.04 (Focal)
The correct setting seems to be `PrivateMounts=no` . At least it worked for me.
Source: https://unix.stackexchange.com/questions/330094/udev-rule-to-mount-disk-does-not-work/441588#441588