This is not an Armbian issue, it's a Le Potato problem and pretty much every single image and distro for this board would do this. Even Libre's images would reset endlessly until they fixed it in newer images, I think sometime last year. CoreELEC is the only OS that would shutdown correctly, EmuELEC worked properly too but that's because it's built from CoreELEC. I was told it's because they use the vendor SDK rather than standard Linux.
The proper command is "sudo shutdown -H now". Libre's solution is to flash the Libre bootloader onto your sd card/emmc overwriting the original bootloader. I can confirm that I did get proper shutdowns when I did this to an old Armbian Focal image I used a while back. It also worked with Batocera and Libre's older releases as well. You can do this with the Libre Computer Flash Tool here https://github.com/libre-computer-project/libretech-flash-tool if you find it annoying.
