Hi,
Same problem for me.
I have Centos-7 and Ubuntu-18 VM, They work fine after the installation.
But after few days (due to the backup i guess), I am unable to boot the VM (No bootable Media Found).
After checking with Rescue Linux, i found that my partition table was erased.
At the beginning I thin that it was due to a misconfiguration in the VM. But the same behavior on the 2 VM make me think that the problem is on Proxmox.
Please let me know if someone found a solution