KVM / libvirt disk consolidation

using ubuntu 20.04 as kvm host, initializing a guest using cloud-init. this gives me a working guest, with an image chain as a result. to simplify the backup process and re-use an existing script, I would like to consolidate the image chain to a single file.

https://libvirt.org/kbase/merging_disk_image_chains.html#consolidating-an-entire-disk-image-chain-into-a-single-image

initial config

virsh domblklist test
 Target   Source
-----------------------------
 vda      test-rootfs.qcow2
 hdd      -

consolidate images

virsh blockcommit test vda --verbose --pivot --active
Block commit: [100 %]
Successfully pivoted

result

virsh domblklist test
 Target   Source
------------------------------------------
 vda      /data/libvirt/disk-images/test
 hdd      -

This is the expected result. Whenever I shutdown the VM and start again, the image config changes to:

virsh domblklist test
 Target   Source
-----------------------------
 vda      test-rootfs.qcow2
 hdd      -

and the consolidated image is not used any more, instead it is using the original image chain again ... -> My Question: Why?

BR and thx for hints

//edit:

I have disabled cloud-init in the guest, then the shudtown / start procedure.

Here is the diff, comparing after block commit and after shutdown / start the domain (using virsh dumpxml).

$ diff 0_blockcommit 1_shutdown_start 
1c1
< <domain type='kvm' id='8'>
---
> <domain type='kvm' id='9'>
32c32
<     <disk type='file' device='disk'>
---
>     <disk type='volume' device='disk'>
34,35c34,39
<       <source file='/data/libvirt/pools/disk-images/test.qcow2' index='3'/>
<       <backingStore/>
---
>       <source pool='disk-images' volume='test-rootfs.qcow2' index='2'/>
>       <backingStore type='file' index='3'>
>         <format type='qcow2'/>
>         <source file='/data/libvirt/pools/disk-images/test.qcow2'/>
>         <backingStore/>
>       </backingStore>
42c46,47
<       <source index='4'/>
---
>       <source file='/data/libvirt/pools/disk-images/test.iso' index='1'/>
>       <backingStore/>
89c94
<       <source mode='bind' path='/var/lib/libvirt/qemu/channel/target/domain-8-test/org.qemu.guest_agent.0'/>
---
>       <source mode='bind' path='/var/lib/libvirt/qemu/channel/target/domain-9-test/org.qemu.guest_agent.0'/>

Workaround:

DOMAIN_NAME="test"
## save xml and remove domain
#
virsh dumpxml ${DOMAIN_NAME} > ${DOMAIN_NAME}.xml
virsh undefine ${DOMAIN_NAME}
virsh shutdown ${DOMAIN_NAME}
 
## redefine domain and start again
#
virsh define ${DOMAIN_NAME}.xml
virsh start ${DOMAIN_NAME}

A ticket was created on the libvirt project: https://gitlab.com/libvirt/libvirt/-/issues/240