Workaround
I used mkusb installed in 18.04, (grub 2.02), to make the foundation of an ISO booter.
ISO's in an ISO folder on usbdata partition, with casper-rw partition for persistence, (one distro only).
Usbboot partition has loop mounting grub.cfg.
Boots 18.04 and 19.10, both UEFI and BIOS, (I got a new power brick for my UEFI computer).
UEFI boot fails using mkusb installed in 19.10, (grub 2.04). whether 18.04 or 19.10.
Workaround
I used mkusb installed in 18.04, (grub 2.02), to make the foundation of an ISO booter.
ISO's in an ISO folder on usbdata partition, with casper-rw partition for persistence, (one distro only).
Usbboot partition has loop mounting grub.cfg.
Boots 18.04 and 19.10, both UEFI and BIOS, (I got a new power brick for my UEFI computer).
UEFI boot fails using mkusb installed in 19.10, (grub 2.04). whether 18.04 or 19.10.
I think the problem is grub 2.04 myself.