I find path to be one of the issues.
As system is not yet mounted with grub, it is defaults as seen if using live installer, but not if you have booted and mounted using your own mount.
I have a partition (hd0,7) and in that partition I have my data partition. While data partition once booted is /mnt/data, before mount it just has a bunch of folders, one of which is ISO. So grub will see it as (hd0,7)/ISO. If you do a ls of that from grub's c for command line then you should see your ISO & if you create a textfile for grub stanza, you see that file. The file has to have correctly formatted grub boot stanza, just like you have in 40_custom.
So is your data partition the hd0,7?
and is path to ISO this? /Iso/-amd64/ubuntu-mate-18.04.4-desktop-amd64.iso"
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