UGH.

I hate when I have to debug an issue and find out its somewhere entirely else, than I was looking.

>Installs a virtual server on the Proxmox VE platform
>Reboots and grub be like "No such device *UUID*"
Whut?

Okay, so... reinstall grub, maybe a bug in the automated install?
>Nop.exe, still an issue
Huh...
>Partition tables all good, drives all visible when booted from a live environment, grub is up to date
>Finally gives up and goes to mess in the (terrible) grub rescue environment
>Grub only sees (hd0) while root is on (hd2), what?
>A whole lot of cursing ensues, wtf?

Turns out it was a bug, but not in grub... Rather, in the QEMU-KVM agent daemon, wtf!

I never had to deal with a bug in the virtualization agent itself.

Downgrading from pve-qemu-kvm 5.0.x to 4.0.x solved the issue.

Now, maybe, I can finally go have my lunch...