> This works in all previous releases I have, from Emacs 20.7 through
> Emacs 25.3.1.

Did it? I don't have a release of 25.3 at hand, but I have one of 24.5 and the effect is the same.

Which isn't surprising, as runemacs' only use is to start an Emacs hidding the console window.

Why don't you do the shortcut with just emacs.exe, instead of runemacs.exe?