Who would go to the trouble of making such a mask though? And how would they do it without you knowing something was up?
As Wired pointed out in their article on their failed attempts at making a mask to get into the iPhone X, "Who is dumb enough to allow a crook to dump several pounds of silicone mask mould on their face without their cooperation or fail to notice it if they aren't cooperating?"
When they were describing the process, they showed it took several hours to mould the face, then lots more hours of effort to make the masks. . . days in fact. By that time the iPhone X's capability to USE FaceID of its user would have expired.