Comment # 5 on bug 1127481 from Josef Reidinger

Well, it looks like DVD is boot with EFI. And bootloader prints warning about
it, at least it is what I see in logs:

proposal returns warning with level warning and msg Cannot upgrade the
bootloader because of a mismatch of the boot technology. The upgraded system
uses <i>Legacy BIOS boot</i> while the installation medium has been booted
using <i>EFI boot</i>.<br><br>This scenario is not supported, the upgraded
system may not boot or the upgrade process can fail later.

So from my POV it works as expected. It should print warning on proposal screen
( I need to test if it print also additional confirmation during upgrade
confirmation ). So looks like DVD is accidentally booted via EFI and not legacy
( or kernel wrongly report it as EFI ).