Comment 47 for bug 1916485

Guest is Hirsute to have
  libc6:s390x 2.33-0ubuntu5 s390x

The following (not optimized for speed but readability) gives us a simple environment-matrix for comparisons:

for r in xenial bionic focal groovy hirsute; do
    uvt-simplestreams-libvirt --verbose sync --source http://cloud-images.ubuntu.com/daily arch=s390x label=daily release=${r};
    uvt-kvm create --host-passthrough --password=ubuntu ${r}-test-nspawn release=${r} arch=s390x label=daily;
    uvt-kvm wait ${r}-test-nspawn
    uvt-kvm ssh ${r}-test-nspawn "sudo apt update"
    uvt-kvm ssh ${r}-test-nspawn "sudo apt upgrade -y"
    uvt-kvm ssh ${r}-test-nspawn "wget https://cloud-images.ubuntu.com/hirsute/current/hirsute-server-cloudimg-s390x-root.tar.xz"
    uvt-kvm ssh ${r}-test-nspawn "mkdir h"
    uvt-kvm ssh ${r}-test-nspawn "sudo tar -xzf hirsute-server-cloudimg-s390x-root.tar.xz -C h";
    uvt-kvm ssh ${r}-test-nspawn "sudo apt install -y systemd-container"
    uvt-kvm ssh ${r}-test-nspawn "sudo reboot";
    sleep 5s
    uvt-kvm wait ${r}-test-nspawn
done

Test (as before) is:
$ cd h
$ sudo systemd-nspawn
$ bash -c 'test -x /usr/bin/gpg || echo Fail'

Out of that I can confirm (and further limit releases to just bionic) that on s390x we have:
Xenial - 4.4.0-210/229-4ubuntu21.31 - works
Bionic - 4.15.0-142/237-3ubuntu10.46 - fails
Focal - 5.4.0-72/245.4-4ubuntu3.6 - works
Groovy - 5.8.0-50/246.6-1ubuntu1.3 - works
Hirsute - 5.11.0-16/247.3-3ubuntu3 - works

Next I split the Bionic case to for a few usual suspects (kernel/systemd/glibc):
Already on step #1 Kernel I found something:
Bionic - 4.15.0-142/237-3ubuntu10.46 - fails
Bionic - 5.4.0-72/237-3ubuntu10.46 - works

Ok so the new kernel fixes it (whatever it is) so what about things in proposed already:
There is a new 4.15 kernel and a new systemd
Bionic - 4.15.0-143/237-3ubuntu10.47 - fails

So none of the builds in proposed has the fix, but something between 4.15 and 5.4 kernels fixes it. That might also be the reason why the other releases are fine - the kernel levels are either >=5.4 (fixed) or <4.15 (not having the issue).