When booting an Ubuntu installer ISO (eg from a USB-stick) on an EFI-capable machine, it may sometimes happen that it still boots the legacy-BIOS version. I finally found out *why* this happens - and how to change that.
When you select the boot-device in the BIOS, make sure it boots the medium (USB-stick) in "UEFI" mode.
Often, the same boot-medium is displayed *twice* in a list to choose from - and one is legacy, the other one is UEFI.
(x)ubuntu ISO: boot in efi/non-efi mode
(x)ubuntu ISO: boot in efi/non-efi mode
Jumping out of an airplane is not a basic instinct. Neither is breathing underwater. But put the two together and you're traveling through space!