If host goes prone everygone goes prone but anybody else goes prone only the person that does so goes prone.
However, everyone appears to go prone when the local (remote) user does (model changes).

Doh! forgot to check that the player was the local client player, so if the keybind was down, it was setting all the players to prone. On the client it was just visual (the client has no say), but on the server (i.e. host) it was setting all players to prone when the host had the keybind down!