Comment # 39 on bug 1047385 from Chris Ward

I ran an 'install' instead of an 'upgrade' today, and the problem went away ---
the option to boot Windows appeared in the grub menu. So the problem was
presumably that one of the tools involved in boot setup was inconsistently
updated.

>From memory, the machine was installed with Tumbleweed a couple of years ago,
and then updated with Leap 43.2 just over a month ago. I don't think I'd be
able to reproduce the install/upgrade sequence leading to the problem, so I'm
closing the bug as 'unreproducable'.