Hey @Gilfoyle - I remember trying to help out in Discord on this one. Guessing the pool is still unmountable?
Proxmox isn’t supposed to grab pools without the cachefile set to its own /etc/zfs/zpool.cache path (TrueNAS uses /data/zfs/zpool.cache) but Proxmox also appears to have a system service that will import pools on disk scan. So if there’s something that triggers a disk scan on a host and the member disks are visible this is likely to cause the simultaneous access/force-import bug. Isolating the disks via PCI passthrough of the HBA is the only way to guarantee that Proxmox can’t touch them once the TrueNAS VM is booted - but it may still attempt to grab the pool during Proxmox host boot unless the HBA is fully blacklisted from the host.
Setting systemctl disable --now zfs-import-scan.service on the host can disable this “import scan” behavior but it’s likely going to need some deeper investigation and soak-testing to determine when and where Proxmox might still try to grab the pool.
(Virtualizing TrueNAS on VMware isn’t impacted by this, because for better or worse, VMware doesn’t speak ZFS.)