MacWorld reports that Apple has secretly added a new Touch ID rule in iOS. The new rule requires a user to enter a passcode to unlock their iPhone or iPad when the device has not been unlocked using a passcode for six consecutive days and has not been unlocked using Touch ID in the last eight hours.
Up until now, iOS prompted users to use the passcode for unlocking purposes when the device was restarted or switched on, has not been unlocked for over 48 hours, was remotely locked using Find My iPhone, and after five unsuccessful Touch ID unlocking attempts. iOS also prompts users to enter the passcode while adding a new fingerprint to Touch ID.
This is likely going to be a pretty common scenario that many iPhone and iPad owners are going to face, since the Touch ID sensor is very accurate and reliable, which makes entering passcode to unlock and iOS device a redundant method. I almost exclusively use Touch ID to unlock my iPad and the device does sit idle for more than 8 hours at a stretch without me using it.
Apple says that it added this new Touch ID rule with the release of iOS 9 last year itself, but the change only appeared in the iOS Security Guide on May 12, 2016. It remains unclear what prompted Apple to take this decision. Nonetheless, its a minor change and most iOS users are likely never going to notice it.