"NotAutomatic: yes" does not affect source packages

I hope I’m not completely OT asking this here:

I just learned that the Debian backports repos are marked "NotAutomatic:
yes" (and “ButAutomaticUpgrades: yes”), which is really handy as it
allows to always keep a suitable backports entry in sources.list, while
still being able to only install _/specific/_ backports packages by
using apt’s -t command-line switch. But while this works just fine for
binary packages, it seems to fail for source packages: They are treated
as if the backports repo is just an ordinary repo, i.e. an “apt-get
source –download-only <some-package>” always downloads the (more recent)
package from the backports repo _/despite/_ the fact that it is
“NotAutomatic: yes”.

Can anyone tell why source packages are not treated the same as binary
packages for “NotAutomatic: yes” repos? It breaks the expectation that
you always get the corresponding sources for a binary package.

The session below shows the issue using a Debian bullseye (snapshot)
chroot, showing that source- and binary versions do not match:

# cat /etc/apt/sources.list
deb https://snapshot.debian.org/archive/debian/20230518T025801Z bullseye
main
deb https://snapshot.debian.org/archive/debian/20230518T025801Z
bullseye-updates main
deb https://snapshot.debian.org/archive/debian/20230518T025801Z
bullseye-backports main
deb https://snapshot.debian.org/archive/debian-security/20230517T222456Z
bullseye-security main
deb-src https://snapshot.debian.org/archive/debian/20230518T025801Z
bullseye main
deb-src https://snapshot.debian.org/archive/debian/20230518T025801Z
bullseye-updates main
deb-src https://snapshot.debian.org/archive/debian/20230518T025801Z
bullseye-backports main
deb-src
https://snapshot.debian.org/archive/debian-security/20230517T222456Z
bullseye-security main
# ls
# apt -o Acquire::Check-Valid-Until=false  update
Hit:1 https://snapshot.debian.org/archive/debian/20230518T025801Z
bullseye InRelease
Hit:2 https://snapshot.debian.org/archive/debian/20230518T025801Z
bullseye-updates InRelease
Hit:3 https://snapshot.debian.org/archive/debian/20230518T025801Z
bullseye-backports InRelease
Hit:4
https://snapshot.debian.org/archive/debian-security/20230517T222456Z
bullseye-security InRelease
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
All packages are up to date.
# apt-get download e2fsprogs
Get:1 https://snapshot.debian.org/archive/debian/20230518T025801Z
bullseye/main amd64 e2fsprogs amd64 1.46.2-2 [615 kB]
Fetched 615 kB in 1s (1109 kB/s)
# apt-get source --download-only e2fsprogs
Reading package lists... Done
NOTICE: 'e2fsprogs' packaging is maintained in the 'Git' version control
system at:
https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git -b debian/master
Please use:
git clone https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git -b
debian/master
to retrieve the latest (possibly unreleased) updates to the package.
Need to get 9714 kB of source archives.
Get:1 https://snapshot.debian.org/archive/debian/20230518T025801Z
bullseye-backports/main e2fsprogs 1.46.6-1~bpo11+1 (dsc) [2762 B]
Get:2 https://snapshot.debian.org/archive/debian/20230518T025801Z
bullseye-backports/main e2fsprogs 1.46.6-1~bpo11+1 (tar) [9624 kB]
Get:3 https://snapshot.debian.org/archive/debian/20230518T025801Z
bullseye-backports/main e2fsprogs 1.46.6-1~bpo11+1 (asc) [488 B]
Get:4 https://snapshot.debian.org/archive/debian/20230518T025801Z
bullseye-backports/main e2fsprogs 1.46.6-1~bpo11+1 (diff) [86.7 kB]
Fetched 9714 kB in 46s (210 kB/s)
Download complete and in download only mode
# ls
e2fsprogs_1.46.2-2_amd64.deb           e2fsprogs_1.46.6-1~bpo11+1.dsc
e2fsprogs_1.46.6.orig.tar.gz.asc
e2fsprogs_1.46.6-1~bpo11+1.debian.tar.xz e2fsprogs_1.46.6.orig.tar.gz