Yeah, I was about to say when I read the first part. Adding windows manually is quite an old thing. Modern linux setups with grub2 will "find" the EFI loader for windows and add it automatically.
I lost my grub loader when I upgraded hardware recently. But, I just booted into a linux USB, chroot (remembering to mount /boot/efi) and re-run grub install/grub update. That didn't find windows. But, it was fine because I just properly booted into linux and ran grub-update again there, and it found it fine.