TouchID requires your Password after 48 hours
The first part of your question has to be a glitch. The iPhone 6S is still a little buggy; mine doesn't always play system sounds, like the Siri chime and the lock "click" sound. But I can explain the second part of your questing:

Apple thinks that a passcode is more secure than TouchID is due to the fact that the odds of TouchID giving a false-positive are 1 in 50,000 (stated by Apple). However, the article hasn't been updated after the 2nd-Generation TouchID in the iPhone 6S came out, and given its great speed improvements, I would assume that the 2nd-Gen has a lower chance of giving a false-positive than the 1st-Gen. The odds of guessing a 6-digit passcode, would be literally 1 in 1,000,000. Even the odds of guessing right with 5 guesses, 1 in approximately 200,000 is still higher than TouchID. I like the convenience of TouchID and will still use it everyday, but from a purely statistical standpoint, a passcode is much more secure.