I ran into this yesterday while installing Bullseye on a UEFI system
with multiple drives and ESPs. Here are a couple of links describing how
Ubuntu's multi-ESP works:
https://unix.stackexchange.com/questions/621942/mirroring-efi-system-partition-esp-on-ubuntu
https://bugs.launchpad.net/ubuntu/+source/grub-installer/+bug/1466150
I tried running
dpkg-reconfigure grub-efi-amd64
on my multi-ESP Bullseye system in the hope that the Ubuntu support may
have been upstreamed, but alas it has not yet happened. I'll ask on
Ubuntu list as well whether someone is working on upstreaming.
