Comment 1 for bug 1973654

I believe this is caused by debootstrap - it only uses packages from the release pocket (and this is frozen from the time Ubuntu 20.04 LTS was originally released). This is a known issue https://askubuntu.com/questions/744684/latest-security-updates-with-debootstrap but I am not sure if there is much you can do to get debian-installer to say use multistrap instead of debootstrap.