I had many problems with Fastboot not working or displaying "press any key to shutdown".
I find a solution here : https://forum.xda-developers.com/t/help-press-any-key-to-shutdown-in-fastboot.3816021/ In short plug a USB 2.0 Hub into your computer and then plug the phone in one of the hub USB 2.0 ports
I finally succeeded in flashing my Mi A2 with the latest Android image available with MiFlash.