In my case, when I add a 2048GB disk in Foreman for a Proxmox VM, the VM is created correctly and the disk shows as "2T" in proxmox. But then when Foreman re-reads the disk info from Proxmox it must not be able to parse "T" and leaves the size of the disk blank.

The same behavior happens when I create the disk only in Proxmox.

To make any changes (VLAN, Host Group, whatever) I have to delete the disk reference in Foreman, because setting the size will actually extend the existing disk (so 2T because 4T and so on). Thankfully this doesn't delete the disk in Proxmox and the next time you "Edit" you see the same disk with an empty size again.