Every UEFI install overwrites /EFI/ubuntu folder in the ESP - efi system partition. Similar to the way MBR gets overwriten.
Best to fully back up the ESP. But once booted, the partition is hidden with umask=0077 in fstab.
I have posted bug reports to let users change /EFI/ubuntu to something else, but nothing.
You can change name in /etc/default/grub but then it still uses the 3 line grub.cfg in /EFI/ubuntu.
The have hard coded the location of grub.cfg in ESP.
I have installed Bionic twice, and had no issue other than having to edit this back to UUID & partition my main working install is in.
Have not installed for a couple of weeks will do that again, soon.
Code:
search.fs_uuid 5c1e1a3f-261f-4da5-a0c2-8f479b3039de root hd0,gpt2
set prefix=($root)'/boot/grub'
configfile $prefix/grub.cfg