FWIW, not sure if it is relevant, I get a couple of Z processes when its at the MySQL configuring step:
root 751 0.0 0.0 20144 284 pts/0 S+ 22:07 0:00 | \_ /snap/distrobuilder/314/bin/distrobuilder build-lxd ubuntu.yaml .
root 768 0.0 0.8 1165072 15420 pts/0 Sl+ 22:07 0:00 | \_ /snap/distrobuilder/314/bin/distrobuilder build-lxd ubuntu.yaml .
root 776 0.0 0.0 0 0 ? Z 22:07 0:00 | \_ [dirmngr] <defunct>
root 777 0.0 0.0 0 0 ? Zs 22:07 0:00 | \_ [dirmngr] <defunct>
root 780 0.0 0.0 0 0 ? Z 22:07 0:00 | \_ [gpg-agent] <defunct>
root 781 0.0 0.0 0 0 ? Zs 22:07 0:00 | \_ [gpg-agent] <defunct>
root 563 1.3 3.1 68012 59020 pts/0 S+ 22:10 0:01 | \_ apt-get -y install apt-transport-https fuse language-pack-en openssh-client openssh-server mysql-server vim cloud-init
root 2467 0.5 0.2 9108 4712 pts/0 S+ 22:10 0:00 | | \_ /usr/bin/dpkg --status-fd 34 --configure --pending
root 3254 0.1 0.8 21048 16948 pts/0 S+ 22:11 0:00 | | \_ /usr/bin/perl -w /usr/share/debconf/frontend /var/lib/dpkg/info/mysql-server-8.0.postinst configure
root 3259 0.1 0.2 5912 3868 pts/0 S+ 22:11 0:00 | | \_ /bin/bash /var/lib/dpkg/info/mysql-server-8.0.postinst configure
root 3606 0.0 0.0 4260 592 pts/0 S+ 22:12 0:00 | | \_ sleep 1
tss 3384 1.1 0.0 0 0 ? Z 22:11 0:00 | \_ [mysqld] <defunct>