Is this really a bug?
In my opinion it is not.
This is just a user fault.
The solution of this problem relies on two basic steps. At first you
need to create your Ubuntu 18.04 bootable flash drive correctly. And
secondly, you need to bootup you machine using the bootable flash drive
in the correct mode (UEFI/CSM/Legacy BIOS). When your system is MBR
Partitioned with legacy BIOS and you try to install Ubuntu in UEFI mode,
failure of the installation process is natural.
You may want to read more:
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Calling 'apt-install grub-efi-amd64-signed' failed
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