It was unfortunately not the inconsistency that is the problem.
I tried to remove the docker installation from snap and followed the guide you linked to. Now it’s installed via apt and the apt repository in the official docker guide.
I experience the same issue.
I did the following:
- Try with exact same swarm setup on the apt installation as I did with the snap installation.
- Removed all worker nodes, so there’s only the manager.
- Tried with both the nfs4 and nfs option.
None of it worked. 
I ran the following:
docker volume create --driver local --name portainer --opt type=nfs4 --opt device=:/volume1/docker_volumes/portainer --opt o=addr=192.168.10.4,rw,nolock
And got the following in the inspect:
$ docker volume inspect portainer
[
{
"CreatedAt": "2020-12-06T21:49:42+01:00",
"Driver": "local",
"Labels": {},
"Mountpoint": "/var/lib/docker/volumes/portainer/_data",
"Name": "portainer",
"Options": {
"device": ":/volume1/docker_volumes/portainer",
"o": "addr=192.168.10.4,rw,nolock",
"type": "nfs4"
},
"Scope": "local"
}
]
I still see an empty directory here:
$ sudo ls -la /var/lib/docker/volumes/portainer/_data
total 8
drwxr-xr-x 2 root root 4096 Dec 6 21:49 .
drwxr-xr-x 3 root root 4096 Dec 6 21:49 ..
Could there be anything I haven’t installed? I followed the official installation guide.
Shouldn’t I be able to see the content of the NFS share in the directory /var/lib/docker/volumes/portainer/_data ?