I had the same problem (twice! in 3 years), on my backup server, where I pull zfs snapshots from my main server.
The critical part is to set -uand -o mountpoints=none during zfs receive, so that the remote root pool that is mounted to / is not...
I also saw this warning during the proxmox 8 to 9 upgrade:
```
Configuration file '/etc/initramfs/post-update.d/systemd-boot'
==> Modified (by you or by a script) since installation.
==> Package distributor has shipped an updated version...