If you've been noticing that you need to enter your passcode to unlock your phone more often, instead of being allowed to do so with Touch ID, you're not alone. With iOS 9, Apple apparently made a security change that it's only recently documented, which may require you to enter your passcode more often before you can use Touch ID again.
The change first popped up in Apple's May 2016 iOS Security Guide:
The passcode has not been used to unlock the device in the last six days and Touch ID has not unlocked the device in the last eight hours.
As noted by MacWorld, while people seem to have been encountering this security feature more frequently in the last few weeks, Apple says that it was added last fall with the launch of iOS 9. What Apple has not made clear is a reason for the change.