Comment 14 for bug 1887535

for verifications:

checking the source build from a git checkout, which is where the problem was, I verified i could reproduce the issue with the -updates source versions:

$ ls -ld .git
drwxrwxr-x 7 ubuntu ubuntu 4096 Apr 7 19:06 .git
$ dpkg-parsechangelog -S Version
1:4.2-3ubuntu6.14
$ dpkg-buildpackage
...

after the 'configure' step has finished, while the build is running (or after it completes), check the build dir for the setting of GIT_UPDATE
$ grep -r GIT_UPDATE b
b/s390xfw/config-host.mak:GIT_UPDATE=yes
b/qemu-xen/config-host.mak:GIT_UPDATE=yes
b/qemu-microvm/config-host.mak:GIT_UPDATE=yes
b/qemu/config-host.mak:GIT_UPDATE=yes
b/user-static/config-host.mak:GIT_UPDATE=yes

since we *do not* want the build to try to automatically update the git submodules, this setting is wrong.

$ ls -ld .git
drwxrwxr-x 7 ubuntu ubuntu 4096 Apr 7 19:33 .git
$ dpkg-parsechangelog -S Version
1:4.2-3ubuntu6.15
$ dpkg-buildpackage
...

for the -proposed version, GIT_UPDATES has been replaced with GIT_SUBMODULES_ACTION, which should be set to 'ignore'
$ grep -r GIT_UPDATE b
$ grep -r GIT_SUBMODULES_ACTION b
b/s390xfw/config-host.mak:GIT_SUBMODULES_ACTION=ignore
b/qemu-xen/config-host.mak:GIT_SUBMODULES_ACTION=ignore
b/qemu-microvm/config-host.mak:GIT_SUBMODULES_ACTION=ignore
b/qemu/config-host.mak:GIT_SUBMODULES_ACTION=ignore
b/user-static/config-host.mak:GIT_SUBMODULES_ACTION=ignore

for the source build from a non-git checkout, this has not been a problem, so verification only requires checking there are no regressions:

$ dpkg-source -x qemu_4.2-3ubuntu6.14.dsc
...
$ cd qemu-4.2/
$ ls -ld .git
ls: cannot access '.git': No such file or directory
$ dpkg-parsechangelog -S Version
1:4.2-3ubuntu6.14
$ dpkg-buildpackage
...

$ grep -r GIT_UPDATE b
b/s390xfw/config-host.mak:GIT_UPDATE=no
b/qemu-xen/config-host.mak:GIT_UPDATE=no
b/qemu-microvm/config-host.mak:GIT_UPDATE=no
b/qemu/config-host.mak:GIT_UPDATE=no
b/user-static/config-host.mak:GIT_UPDATE=no
$ grep -r GIT_SUBMODULES_ACTION b
$

$ dpkg-source -x qemu_4.2-3ubuntu6.15.dsc
...
$ cd qemu-4.2/
$ ls -ld .git
ls: cannot access '.git': No such file or directory
$ dpkg-parsechangelog -S Version
1:4.2-3ubuntu6.15
$ dpkg-buildpackage
...

$ grep -r GIT_UPDATE b
$ grep -r GIT_SUBMODULES_ACTION b
b/s390xfw/config-host.mak:GIT_SUBMODULES_ACTION=ignore
b/qemu-xen/config-host.mak:GIT_SUBMODULES_ACTION=ignore
b/qemu-microvm/config-host.mak:GIT_SUBMODULES_ACTION=ignore
b/qemu/config-host.mak:GIT_SUBMODULES_ACTION=ignore
b/user-static/config-host.mak:GIT_SUBMODULES_ACTION=ignore

additionally, the build is successful with the version from -proposed for both a non-git source and git source.