Comment 9 for bug 1868138

/etc/default/grub is ucf/debconf managed conffile, which has always been processed by grub2 maintainer scripts, and there was no change in behaviour of that in bionic.

You may not have ever noticed this before, because you might have only applied the config change without re-running dpkg-reconfigure on the relevant grub2 package, nor apply any updates that might be available. Note that on all new installations / launches of new instances, all updates are applied during installation process where possible. Thus this is not a regression in updates.

Many of the options specified there are not used by Ubuntu initramfs-tools and have no effect, why are you specifying dracut options? Have you converted your system from initramfs-tools to dracut?

Did you experience that the syntax used in the conffig file ever worked across grub2 upgrades or dpkg-reconfigure in any Ubuntu or Debian release?

Note, that you can always use dpkg preseed of the debconf key with all of the extra commandline options which will persist across grub2 updates; or use the .d drop in files (although those too, get parsed).