I wanted to stop qbittorrent and sonarr dockers and lazily chose to do that via the Prtainer docker. But Synology didn't like that... I got this notification.

Docker container qbittorrentvpn stopped unexpectedly.

I will note that these containers are linked. In Portainer, I first shutdown the linked qbittorrent/sonarr container, followed by the still running qbittorrent container. Portainer seemed content with that.

Have I done something wrong here?

Iam getting the same. Not always, but sometimes.

I also get this error message in the syno UI if I run command 'docker container stop'.

"stopped unexpectedly" from the perspective of the Syno Docker UI... I always felt this is a false negative.

Would the Docker UI register a watch on the Docker event stream, it would actualy receive the termination information... Well, at least this is what their implementation should have used.

An example for good use of watches on the event stream is Traefik.. Whenever a conatiner commes up, it checks its labels and applies those instantly to its reverse proxy rules set. When a labeled container dies, it instantly removes the reverse proxy rules for this container... There is a way... Synology just needs to leverage it...