Years pass, but Apple just wouldn’t fix this bug in iOS. When the iPhone asks you for the passcode, it shows the keyboard where system keys’ borders aren’t semitransparent:
You may think that it is so by design, but no: if you touch the keyboard, all keys’ border re-render normally, with thin borders.
And sometimes the keyboard is rendered fine initially:
Does nobody in Apple use alphanumeric passcodes? The numeric ones you usually see others type from three-meters distance. Since there is Touch ID and the passcode is rarely asked for, there is no reason to use the numeric ones.