Why does 74hc595 work "better" without GRND (pin 8) grounded?

larryd:
Try 10K

No difference. Short blink on, then silent.

As before, with a loop like follows, sending data constantly, I can still only measure 2 millivolt on the data cable. ~120 mV on latch and ~1.3 on clock.

void loop() {
  if (millis() - lastSwap > 2000) {
    current = current == LOW ? HIGH : LOW;
    lastSwap = millis();
    Serial.println(current);
  }
  output(max_d0, current);
}

Output is ofc. HI/LO as expected.
LED stays off.