I’m asking this as a few times when I use FaceID, my phone is a little slow and instead of unlocking straight away I see the password box appear and then the password gets entered and it unlocks.
This leads me to believe that FaceID doesn’t actually unlock the phone it just adds the password on the password screen.
Am I correct in this?
That’s not correct. FaceID unlocks the device. I’ve used it on 2 iPhones and 2 iPads and that’s how it’s worked.
From time to time, the device demands you input your password as a security measure; it also requires your password after a restart.
What you’re experiencing suggests that it’s just not recognising your face very well. You could try turning off :Require Attention for Face ID" in Settings; or Reset Face ID completely and rescan your face.
Also, I seem to remember that Face ID requires more than a simple 4-digit passcode. That might also be a factor.
Finally (should have asked this first, I suppose) what model iPhone and what version of iOS?
The following may provide the info:
Thanks for that, but it’s not really clear what happens. It seems that FaceID is a subsystem. What it doesn’t tell you is if FaceID enters the password for you, or bypasses the password and unlocks the device with FaceID itself.
I should add that this is purely out of curiosity
13 Pro Max, iOS 17.
I’ve seen the same behaviour with my 13 Pro Max too.1
This is also very interesting because I was under the same impression just yesterday, but didn’t pay much attention. As a matter of fact, I cannot remember if it was with my oldish iPhone 8 (TouchID) or with the new iPad (FaceID)… perhaps if FaceID cannot get a clear shot of your face it begins displaying the passcode keyboard and overrides it a few miliseconds when it secures your identity, hence the quick flash…
Face ID unlocks the device itself. It only passes across to the passcode if: it doesn’t match your face to the face “image” it has stored; you have restarted the device; you have reached the limit of face unlocks before the passcode is needed; or you have invoked the action to temporarily disable Face ID.
Biometric ID is by design separated from the passcode
I think you’re misunderstanding what’s happening.
Face ID is unlocking the phone, there is no manual entering of the passcode.
The FaceID icon spins a while then shows the passcode screen and that gets populated automatically, seemingly by FaceID.
Also if you’ve not unlocked your device for more than 24 hours (I think)
FaceID (or TouchID) can only verify your identity; they cannot really unlock your device (Unlock is an ambiguous term here because it means something different depending on whether you’re starting your device or waking it): There are cryptographic keys that are derived partly from your passcode which are discarded when the phone is turned off. Those keys cannot be recovered by FaceID which is why you have to enter your passcode after restarting your device.
I think the OP is asking about the situation that sometimes happens when FaceID seemingly fails to recognize you, displays a passcode prompt, then decided that it is actually you, and seemingly fills in the passcode prompt for you.
It’s possible that the FaceID system tells the authentication system that you are you and to go ahead and fill in the passcode. From a software design point of view it would be a nice way to deal with the passcode prompt. From a security point of view, I’m not sure that I like the idea that any system in the device retains my passcode after it’s been used to derive whatever keys are derived from it.
tl;dr I don’t really know but that’s my take on it 
And sometimes it’s not that it fails, but it seems a little slow in unlocking.
Just to clarify, I’m not talking about a FaceID failure, but when it unlocks it sometimes shows the passcode box and fills it in itself to unlock the phone.
This causes me to wonder whether FaceID simply stores your passcode and then enters it into the passcode box when you use FaceID. Usually it happens so fast you don’t see the process.
My comment above has led me to a similar question.
On my iPhones ( 2 of them) and my iPads (2 of them), the password screen doesn’t show (except (a) in the circumstances I described earlier and (b) if, as occasionally happens, Face ID is unusually slow.
This could be like the dial tone of a landline phone; serves no purpose other than to let the user know the phone is working. Maybe it’s not and this is just more of a user experience thing. No idea…
I played with this a bit and I think the animation is just pretending to enter the passcode or paste the password if you use all characters. Are you seeing it actually highlight the keypad numbers in order? Would be interesting to capture with high speed video…