Linux Lite 5.4 Final Released

[Valtam]

The conclusion is that the script has some bug at the "GRUB" writing stage. I assume the build of the grub.cfg is somewhere wrong and when the update-grub is issued, the wrong file is written over the old one.
Hope for an easy fix...
All the best!

There is no 'problem with the script' that I can see. I also have UEFI firmware and my GRUB is on  /dev/sda1 device. The "/" partition is /dev/sda2 as well. No issues here when I upgraded, when the upgrade instructions are followed to the letter.