Apple has a feature to restrict changing the passcode when not in a typical geolocation. This is to prevent thieves from shoulder-surfing your passcode in a bar and then snatching the phone and changing the code. If you’re at home or at work (where the phone recognizes your typical behavior / location), it shouldn’t activate. In a random location, it would. I don’t know how well that works. It’s a relatively recent feature (within a year or so, I think).