Electronic – arduino – Direct port manipulation gives random result: Delay necessary

Setup (Arduino Pro Mini ATmega328p 16MHz, 5V, no pin connected):

When called from the setup(), this works:

pinMode(11, INPUT_PULLUP);
bool identifiesAsFast = digitalRead(11); // always true

This doesn't:

const uint8_t identificationPinBitNumber = PB3; // pin 11
const byte identificationPinBit = bit(identificationPinBitNumber);
DDRB &= ~identificationPinBit;
PORTB |= identificationPinBit;
bool identifiesAsFast = PINB & identificationPinBit; // occasionally false

Pin 11 is not connected, but – as you can see above – I have it pulled high. So I would expect it to always have a value of 1.

What am I missing when doing direct port manipulation?

I get the behavior with four different Arduino Pro Minis. All are original Pro Minis, designed by Sparkfun, made in the USA and bought from Digi-Key. One of them I pulled freshly out of the original bag. I may have to remove and insert the FTDI adapter around thirty times, but eventually I am able to reproduce the issue.

Adding a delay of 100 ms in between pulling pin 11 high and reading it seems to resolve the issue. However, I don't understand why that delay would be necessary.

PB3 for pin 11 is correct, as I verified by connecting an LED and making it blink. See also the pin mapping diagram which can be found elsewhere on the web:

Full code:

// Felix E. Klee <felix.klee@inka.de>

const uint8_t ledPinBitNumber = PB5; // pin 13
const byte ledPinBit = bit(ledPinBitNumber);

const uint8_t identificationPinBitNumber = PB3; // pin 11
const byte identificationPinBit = bit(identificationPinBitNumber);

unsigned long delayDuration; // ms

void turnLedOn() {
  PORTB |= ledPinBit;
}

void turnLedOff() {
  PORTB &= ~ledPinBit;
}

void setDelayDuration() {
#if 0
  pinMode(11, INPUT_PULLUP);

  bool identifiesAsFast = digitalRead(11); // always true
#else
  // Input pullup:
  DDRB &= ~identificationPinBit;
  PORTB |= identificationPinBit;

  bool identifiesAsFast = PINB & identificationPinBit; // occasionally false
#endif

  delayDuration = identifiesAsFast ? 300 : 3000;
}

void setup() {
  DDRB |= ledPinBit; // Set up LED pin for output
  setDelayDuration();
}

void loop() {
  turnLedOn();
  delay(delayDuration);
  turnLedOff();
  delay(delayDuration);
}