Apparently grub-pc.postinst doesn't run grub-install for Trixie anymore.
I used debootstrap to install Trixie on a spare partition of a pretty old
system, told Bookworm's grub on /dev/sda1 to search for other bootable
partitions and ran update-grub, booted into /dev/sda3 (Trixie), ran
"dpkg-reconfigure grub-pc" as usual, but on the next reboot it still
showed the old grub menu found on /dev/sda1.

I had to manually run grub-install.


Regards

Harri