For reference, the problem seems to be that libreoffice-base-core (in main) depends on "libreoffice-core-nogui | libreoffice-core", and the former is in universe whereas the latter is in main.
apt handles this gracefully, I am able to install libreoffice-base-core in a focal chroot without universe. The ISO builder doesn't seem to handle this well, though.
For reference, the problem seems to be that libreoffice- base-core (in main) depends on "libreoffice- core-nogui | libreoffice-core", and the former is in universe whereas the latter is in main.
apt handles this gracefully, I am able to install libreoffice- base-core in a focal chroot without universe. The ISO builder doesn't seem to handle this well, though.