Comment 3 for bug 1847105

Interesting - at first the only related change seems to be in libvirt.
Function storageBackendCreateQemuImgOpts of src/storage/storage_util.c.
It is in there since [1] which on itself makes sense to me.
Also that particular change is in libvirt since v4.3 which means >=Cosmic and we haven't had major bugs in those releases nor in e.g. cloud archive backports of the same due to it.

Looking forther points in virt-manager to [2] but that is from 2013, but looking at recent related code shows [3] which is only in since v2.2.0 and that would match.

That is in /usr/share/virt-manager/virtinst/storage.py of binary "virtinst" of src:virt-manager.

From debug we see the config virt-manager defines for that disk:
virt-manager 8272] DEBUG (storage:643) Creating storage volume with xml:
<volume>
  <name>ubuntu19.04.qcow2</name>
  <capacity>16106127360</capacity>
  <allocation>16106127360</allocation>
  <target>
    <format type="qcow2"/>
    <features>
      <lazy_refcounts/>
    </features>
  </target>
</volume>

Which matches what libvirt receives:
debug : virStorageVolCreateXML:1499 : pool=0x2867ae0, xmlDesc=<volume>
  <name>ubuntu19.04.qcow2</name>
  <capacity>16106127360</capacity>
  <allocation>16106127360</allocation>
  <target>
    <format type="qcow2"/>
    <features>
      <lazy_refcounts/>
    </features>
  </target>
</volume>
, flags=0x1

[1]: https://libvirt.org/git/?p=libvirt.git;a=commit;h=c9ec7088c7a3f4cd26bb471f1f243931fff6f4f9
[2]: https://github.com/virt-manager/virt-manager/commit/37350859ce6c46a1c7e6562db8e242a97bcf5bb3
[3]: https://github.com/virt-manager/virt-manager/commit/4f66c423f7833e270b61536d53a0772ce1242abc