What happens when Serial1 is directly connected to Serial2/3 on Mega 2560?

Projects Networking, Protocols, and Devices
17

This is the exact code that was producing the funny y on my board (and it still is):

void setup() {
  // initialize serial:
  Serial.begin(9600); // For monitoring on PC
  Serial1.begin(9600); // Sender
  Serial3.begin(9600); // Receiver
}

void loop() {
  Serial1.print('A'); // Send from Serial1 to Serial3, so wire them up accordingly, directly, without anything like a MAX232
  delay(2000);
  
  if (Serial3.available()) {
    // get the new byte:
    char inChar = (char)Serial1.read();
    Serial.println(inChar); // Send to PC what Serial3 received
  }
}

I've even power off/reset the board several times by now.