Comment 28 for bug 1949089

This added BindsTo is what's causing the problem:

ddstreet@localhost:~$ systemctl show -p BindsTo snap-hello\\x2dworld-29.mount
BindsTo=
ddstreet@localhost:~$ sudo systemctl daemon-reload
ddstreet@localhost:~$ systemctl show -p BindsTo snap-hello\\x2dworld-29.mount
BindsTo=dev-loop6.device

ubuntu@test-b-full:~$ systemctl show -p BindsTo snap-hello\\x2dworld-29.mount
BindsTo=
ubuntu@test-b-full:~$ sudo systemctl daemon-reload
ubuntu@test-b-full:~$ systemctl show -p BindsTo snap-hello\\x2dworld-29.mount
BindsTo=

Once the mount unit BindsTo= the loop device, systemd won't start the mount unit (which actually runs 'mount') until the loop device is 'started' but of course the loop device won't ever be started until 'mount' is called.