Why the problem has resolved, I don't know for sure. I did run grub2-mkconfig -o /boot/grub2/grub.cfg, but I did that the last time. I only changed one thing, and noticed another.
- What I noticed was that this update included grub2-common-1:2.02-90.el8.noarch. Did this new release of grub2-common make a difference? I don't know.
- What I changed was that I got rid of the symbolic link that ensured /boot/grub2/grubenv and /boot/efi/EFI/centos/grubenv were the same, and instead used identical copies in both places. I was already doing this with /boot/grub2/grub.cfg and /boot/efi/EFI/centos/grub.cfg, as I had found my system failed reboot if I used a symbolic link there. Is this what made the difference? I don't know that either.