Upgrading Helios64 from Armbian Buster to Bullseye (see below) works as expected on my system.
However, I am using systemd-networkd and just a few services (nextcloud, netatalk, etc. and not ZFS)
EDIT: Upgrading Buster installations to Bullseye also works fine if you use network-manager, even if you have a bridge configured (using bridge-slave; binutils-bridge).
# cat /etc/os-release
PRETTY_NAME="Debian GNU/Linux 11 (bullseye)"
NAME="Debian GNU/Linux"
VERSION_I
Armbian "current" (5.10.y) compiles without issues.
I second @Igor's opinion that a change somewhere in this diff broke the eMMC.
I tried reverting a few obvious parts of it, like the mmc driver changes, but without success.
However I did find that with the unit I have the issue happens only in hs400{,es} modes.
With those disabled my unit works fine and I can use nand-sata-install to transfer os from SD to eMMC successfully which is not possible with 5.10.60+ and hs40