I ran into this issue (xcp-ng 8.3.0 beta 2, XO 4cf03 ronivay docker image) and thought it had to do with my vm disk storage location (local vs NFS vs iSCSI). After not being able to restore on the file level from delta backups in all three situations I randomly decided to untick the "set up this disk as an LVM group" when installing a test vm which is enabled by default on Ubuntu 24.04 LTS.

The following screenshot is what the storage setup looks like with the default lvm group.