@NeilGrevitt Whilst migrating my other backups to NFS, I noticed that one of the VM backups is able to perform file restores successfully.
The successful VM is Ubuntu 22.04.1 LTS, but has been upgraded multiple times and was originally a flavour of 18.x LTS somewhere along it's life.
The unsuccessful VM is Ubuntu 20.04.5 LTS that I believe was installed at this version.
The only difference I can see is the output of lvscan, which does point to an issue with LVM!
Successful VM:
:~$ sudo lvscan
:~$
Unsuccesful VM:
:~$ sudo lvscan
ACTIVE '/dev/ubuntu-vg/ubuntu-lv' [<19.00 GiB] inherit
Not sure where to go now but thought it worth reporting!