Comment # 18 on bug 1206497 from Stefan Hundhammer

So, you booted into the installation media (a rotational USB disk) in legacy
(non-UEFI) mode. Consequently, it was detected as legacy boot, and the system
was installed accordingly.

If that is not what you wanted, we have a concise guide here:

https://github.com/yast/yast-storage-ng/blob/master/doc/efi.md#on-x86_64-legacy-vs-uefi-booting


I think this may be due to those BIOS settings that you described: 'UEFI and
legacy' and 'legacy first'. This sounds very much like it would normally do an
UEFI boot, but since it detected that USB disk that you used for the Leap 15.4
ISO, for some weird reasons it tried to boot from there in legacy mode.