Are you doing this with your lock screen in the ‘unlocked’ state or in the ‘locked’ state? If the lock screen is already unlocked, it doesn’t work (and wouldn’t matter, since whatever adversary you have already has access to your phone). Try it again with your phone locked and the screen off. If it still doesn’t work, report it as a bug to Apple. I can personally vouch for it working on a 6, 6S, 7 and X.