Some HP Products not honoring EFI/BOOT/grub.conf in custom ISO

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I have an odd situation. I have authored a custom ISO using RHEL 7.8 as the base and modified isolinux/isolinux.cfg and EFI/BOOT/grub.cfg to point to kickstart files located within the ISO. This ISO works as expected on VMware VMs and older HP servers I have. However, I have this HP DL580 GEN9 and a Z440 workstation that do not seem to honor the grub.cfg while attempting an EFI install. Rather I am presented with the default EFI boot screen with the options:

Install Red Hat Enterprise Linux 7.8
Test this media & install Red Hat Enterprise Linux 7.8
Troubleshooting -->

Things to note:

Legacy BIOS mode is generally disabled. However, I performed a one time legacy BIOS boot menu and the ISO properly loaded with the custom kicktart options available.

Also while troubleshooting this with legacy BIOS disabled, while doing a process of elimination, I completely removed the EFI directory and isolinux/isolinux.cfg before compiling the ISO. I still was able to get the default EFI installation menu and start the normal installation process. ( I expected the boot process to fail.)

While trying to figure out where it was still pulling the selection menu from I ran a grep -irHI "troubleshooting" * in the base of my custom ISO path expected it to locate "Troubleshooting -->" and received no results.

My questions:
Why aren't these two systems properly loading the custom EFI/BOOT/grub.cfg in a custom ISO?

How could the EFI installation process work at all having removed the EFI directory and disabling legacy BIOS on the system?

Since the default EFI menu is still loading, is there some kind of default installation menu coded inside a binary somewhere?

Here are my ISO build commands:

mkisofs -o /opt/iso-build/artifact/rhel78-201027v1707.iso -b isolinux/isolinux.bin -c isolinux/boot.cat -U -r -v -T -J -joliet-long -no-emul-boot -boot-load-size 4 -boot-info-table -eltorito-alt-boot -e images/efiboot.img -no-emul-boot -V "RHEL-7.8 Server.x86_64" -volset "RHEL-7.8 Server.x86_64" -A "RHEL-7.8 Server.x86_64" .

isohybrid --uefi /opt/iso-build/artifact/rhel78-201027v1707.iso
CR

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