Arduino Due pin 4 pull-up stuck on?

Official Hardware Mega Due
1

I'm trying to use pin 4 of the Arduino Due as a floating digital input. I have a very simple script:

void setup() {
  pinMode(4, INPUT);
}

void loop() {
}

When this script is run, however, pin 4 still has a pull-up on it. I have to manually switch off the pullup with the register:

void setup() {
  pinMode(4, INPUT);
REG_PIOA_PUDR |= 0x20000000;
}

void loop() {
}

Then it floats as expected.

Pin 5 doesn't have this issue. Anyone know why pin 4 is behaving differently? I've tried both 1.x and 2.x versions of the IDE.

2

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