Comment 5 for bug 1569567

This is expected behavior; it's just under documented =(

curtin writes /etc/default/grub.d/50-curtin-settings.cfg

even if there aren't any extra args from maas due to the maas images which include a default console=ttyS0 setting, forcing serial output even if users don't ask for it. (curtin revno: 98).

Grub has been modified to allow files in /etc/default/grub.d/*.cfg to override /etc/default/grub to allow control without touching /etc/default/grub (ie, leave it pristine).

https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/901600

However, none of this is documented in a user-visible fashion (like insde /etc/default/grub itself).

You may want to reopen grub2 task and request/supply a comment to the default template indicating this.