Systems isn't blocking the shutdown. Systems is being blocked by a service. Since you are saying that you can't track down the exact service, you must be using

Systems isn't blocking the shutdown. Systems is being blocked by a service. Since you are saying that you can't track down the exact service, you must be using a container runtime like docker or LXC which is not transmitting the kill signal properly to the container to shut it down. The container itself then keeps running for whatever reason. This entirely depends on your particular setup and has very little to do with systemd.