I can comment on this. As far as I can tell, giving the "fire" command after it's already been given simply orders the squad to stop firing and return to formation--it's just that Nomad doesn't actually say anything or give a hand signal. I noticed that whenever I give this silent "hold fire" order, my squadmates reattach their suppressors and stop running around in search of targets to shoot. This seems to be it.
How about for the next update/patch, you actually replace the "fire" icon on the command wheel with a "hold fire" icon after the "fire" order has been given, and there's an accompanying verbal order and hand signal from Nomad? That would be more immersive and less confusing.