Started a VM for installation with a qcow2 image as base disk which has backing_file as Debian stable image.

The installer fails to detect that there is an existing OS on the disk (works correctly with Fedora)

The output looks like the following:
[anaconda root@ibm-p8-kvm-03-guest-02 ~]# address=$(cat /run/anaconda/bus.address)

[anaconda root@ibm-p8-kvm-03-guest-02 ~]# busctl --address=$(echo $address) call org.fedoraproject.Anaconda.Modules.Storage /org/fedoraproject/Anaconda/Modules/Storage/DeviceTree org.fedoraproject.Anaconda.Modules.Storage.DeviceTree.Viewer GetExistingSystems
aa{sv} 0

See there are not systems printed.

Mounted the partition with the debian rootfs to check it's contents:

[anaconda root@ibm-p8-kvm-03-guest-02 ~]# mount /dev/vda1 /mnt

[anaconda root@ibm-p8-kvm-03-guest-02 ~]# cat /mnt/etc/fstab 
PARTUUID=02a0854b-0b62-4029-ae42-df0d110f2dc9 / ext4 rw,discard,errors=remount-ro,x-systemd.growfs 0 1
PARTUUID=74510ae0-93c7-4dfe-81fe-de1ea95ffdda /boot/efi vfat defaults 0 0

[anaconda root@ibm-p8-kvm-03-guest-02 ~]# cat /mnt/etc/os-release 
PRETTY_NAME="Debian GNU/Linux 12 (bookworm)"
NAME="Debian GNU/Linux"
VERSION_ID="12"
VERSION="12 (bookworm)"
VERSION_CODENAME=bookworm
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"