I installed Leap 15.0 from Build 124.1 into a new VirtualBox VM with a GPT
partition table. I forced the installation to place the boot information in the
root partition, not in the MBR. The option to write standard boot code into the
MBR was selected. The resulting installation could not boot with the message
that no OS was found.

I booted the Rescue system from the CD and used parted to examine the partition
table on /dev/sda. Everything was as expected except that the "legacy_boot"
flag was not set on the root partition. As soon as I toggled that flag, then
the installation booted as expected.