I also had that issue. I solved it with bind dirs:
$ cat /rw/config/qubes-bind-dirs.d/50_user.conf
binds+=( '/snap' )
binds+=( '/var/cache/snapd' )
binds+=( '/var/lib/snapd' )
then reboot the vm, install the snap package as user, and launch the package at least once before shutting down the vm.
Any other method doesn’t work for me.