Why does Keyboard HID only work when system mode is automatic?

Firmware
2

I’d have to try with a Photon, but AFAICT there is no HID support on mesh devices yet.
It’s a requested feature but not very high on the priority list.

Update:
I have just tried this code on a Photon 1.4.2 and it works as expected.
Each time I hit the SETUP button the expected text gety typed out

SYSTEM_MODE(SEMI_AUTOMATIC)
STARTUP(Keyboard.begin())

void setup() {
}

void loop() {
  if (!digitalRead(BTN)) {
    Keyboard.printlnf("Test %lu", millis());
    delay(1000);
  }
}