I'm having the exact opposite problem you are. GRUB refuses to show up in boot options, or have a menu show up whenever I boot (which is what I want). I want your problem. If I had to suggest a fix you should get into your UEFI firmware and see if GRUB is listed in the boot options then move the Windows boot loader above it. Then Windows should take priority.
My problem is that after I've installed Kali on a seperate partition, GRUB won't show up anywhere and I can't figure out how to boot the new Kali.
