The usual way of using an .ISO file to create a bootable flash drive is to use a program like Etcher or MultiWriter, or even dd. Etcher warns that creating a bootable Windows flash drive requires special handling, so something like Rufus is recommended. I've never seen the process described the way you just described your method.
If it's worked for you in the past, though, I can't suggest why it's not working now.
If you have a Windows machine available to use, perhaps you could use Rufus (free download) to create the USB drive you need? It's never failed for me.
