Restore /etc/default/grub file:
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This will add back missing /etc/default/grub file.
Can help fix things such as plymouth theme not shouwing up or grub foceing itself to display.


sudo apt-get --reinstall install grub-pc


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Source: https://askubuntu.com/questions/406229/there-was-no-etc-default-grub-file-so-how-come-my-system-was-able-to-boot#407311
Date: Febuary 19th 2023.