IANAL. But I believe it's not that easy. First it would have to be a foregone conclusion that you had contraband on your phone before merely locking it could be construed as obstruction. Then they'd have to demonstrate that you locked the phone at a time when you had reason to believe that you might be coerced to unlock it because it was a foregone conclusion that you had contraband. Are those likely circumstances? Probably not, though it will happen to someone, sometime, but when it does it will be because they did something stupid or because the cops overstepped their authority (in the latter case the process is the punishment, and it wouldn't be the cops getting punished).
iPhone periodically disables biometrics until you enter your passcode. They aren’t going to be able to prove that the reason the phone is asking now isn’t that.
