Comment 16 for bug 1873008

On ZFS setup, boot/grub/ uses the EFI system partition to store GRUB configuration so, it's always readable by grub on boot. This partition is of type vfat and this filesystem type doesn't support symbolic links.

The problem with lubuntu grub theme is that a file used for the theme is a symbolic link, so dpkg cannot extract it to the VFAT partition

ls -l src/boot/grub/themes/lubuntu-grub-theme/icons/ubuntu.png
lrwxrwxrwx 1 u u 11 avril 3 10:10 src/boot/grub/themes/lubuntu-grub-theme/icons/ubuntu.png -> lubuntu.png

A solution at the packaging level is to create a copy instead of using a symbolic link.