Same issue here with a Mini S12 Pro (actually 2 of them). Cannot get the devices to legacy boot.
I’ve pulled my hair out googling the issue to death and mucking around in the BIOS. Nothing works.
I’m trying to legacy boot Ubuntu and that fails. And yes the SSD disk is MBR/BIOS and boots fine on other machines in legacy mode. And yes secure boot is off and the boot mode changed to legacy.
FWIW, UEFI boot of a Ubuntu GPT/UEFI SSD disk works.
In both cases, I’m using an external USB drive.
Both devices have BIOS AMI version ADLNV105.
Unless someone has an answer for this issue, I have to conclude that there’s a bug in the BIOS preventing legacy booting.
As such, would a later BIOS version fix the problem?
[The reason I want legacy boot is because I have some existing MBR/BIOS disks (with data) that I want to use in Beelink devices.]