Tripple boot Debian 12, Debian 13, Opensuse EFI

[Richmond <dnomhcir@gmx.com>]

This is using GPT partition layout and UEFI boot.

I had Debian 12 and Opensuse dual booting, with Debian 12 having its own
Grub, and opensuse having its own grub, so I could choose which system
from the UEFI ? firmware settings on grub.

Then I installed a new system running Debian 13. During the installation
it installed grub, but afterwards I could not find Debian 12 anymore. My
attempts to re-install grub didn't work well and I ended up with only
Debian 13 booting without any grub menu, or opensuse booting with
grub. I had to in the end use the usb rescue system to re-install grub
from the Debian 12 system. Why did this all go so wrong? Why would
Debian 13 boot without displaying Grub, even though it had a timeout of
5 seconds?