The Windows 10 ISO contains a file named
sources/install.wim that weights more than 4G, which is the maximum file size on FAT filesystems. So the USB disk must be partitioned using NTFS, however BIOS doesn’t supporting booting from NTFS USB partitions (for whatever reason); this can be worked around by creating the bootable drive using WoeUSB.
nix-shell -p woeusb --run woeusbgui # You might have to set the `$SHELL` environment, and run woeusbgui with `sudo`.
Be sure to enable NTFS support in the kernel first; see https://nixos.wiki/wiki/NTFS
In WoeUSB’s dialog, select “NTFS” as the file system. The resultant USB should properly boot into Windows installer.