Although the bug was first reported in Red Hat Enterprise Linux, apparently related bug reports are rolling in from other distributions from different families as well. Ubuntu and Debian users are reporting systems which cannot boot after installing GRUB2 updates, and Canonical has issued an advisory including instructions for recovery on affected systems.
Although the impact of the GRUB2 bug is similar, the scope may be different from distribution to distribution; so far it appears the Debian/Ubuntu GRUB2 bug is only affecting systems which boot in BIOS (not UEFI) mode. A fix has already been committed to Ubuntu's proposed repository, tested, and released to its updates repository. The updated and released packages, grub2 (2.02~beta2-36ubuntu3.27) xenial and grub2 (2.04-1ubuntu26.2) focal, should resolve the problem for Ubuntu users.
For Debian users, the fix is available in newly committed package grub2 (2.02+dfsg1-20+deb10u2).