Touch ID working for app login but NOT for unlock (iPhone 6S)

For some reason today I was suddenly unable to unlock my iPhone 6S using Touch ID. I updated to iOS 11.1.1 a few days ago but didn't make any changes to my OS or settings today.

I can still use Touch ID to log in to apps, download new apps, pay with Apple Pay, etc.

In Settings -> Touch ID & Passcode, the 'iPhone Unlock' toggle is greyed out, but the Apple Pay and iTunes & App Store toggles are active and turned on / green.

Any ideas about what might be wrong? Thanks!