Why does an iPhone with Touch ID sometimes ask for an unlock code?

 To improve the security of user data, Apple has developed a special protocol to store Touch ID fingerprints. These data are not transmitted to Apple's servers and are stored only on the user's device.

Of course, they are "hidden" in encrypted form and the decryption key is stored only in the iPhone's RAM. To prevent saved fingerprints or encryption keys from being read when you turn off your device, the system wipes them when you turn off your iPhone.

With a new open, the smartphone generates a new key to access the fingerprint, for which it needs an unlock code.

The decryption key is also deleted if the device is left unlocked for a long time or if several consecutive unlock attempts fail. In these cases, you also need to enter the passcode.

It is not possible to disable such checks and it is in the interest of the user to enable it. On the one hand, you turn off and restart your iPhone infrequently, and on the other hand, regular reminders do not make you forget your passcode.