It is admittedly a weaker argument from a pure physical standpoint but the law doesn't operate based upon that anyway. Any judge would throw out a request in discovery for the defendant to produce a recording confessing their guilt even for legitimate comparative purposes and many other requests or demands that would be unduly prejudicial for 5th amendment reasons.
If they have a warrant to check the phone, and if taking fingerprints of the arrested without consent has already been found constitutional, I just don't see a problem.
Consider the password case, you can't be compelled to give your password, but the government is allowed to throw all of their hacking prowess against your device to crack that password. They can even ask you about the name of your first pet to get through security questions. Though they can't compel you to answer truly regarding the name of your first pet.
Or what if a suspects fingerprint was taken without his consent by the police, and compared against a fingerprint taken from the murder weapon? Is that self incrimination?
I'm not seeing the "self incrimination" angle for face or fingerprint scans here. For classical passwords yes, but not for biometrics.