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I have my iPhone SE set to use my fingerprints to access it, and this is normally the way I get into my phone. By at seemingly random intervals (absolutely no pattern that I can detect), the phone will require I enter the actual unlock code. The phone has definitely not been rebooted, crashed, or anything of that sort.
What causes it to just randomly sometimes want the lock code? It happens anywhere from once per week, to once every couple of months, but never with any pattern to the frequency that I can detect.