Communicate ESP32 and PC via MAVLink

Question

I want to have an ESP32 send MAVLink data over serial to a PC. I compiled and uploaded main.cpp onto an ESP32, which for now only sends heartbeat messages. I am using this code for MAVLink.h. main.cpp is as follows:

#include <Arduino.h>
#include <MAVLink.h>

void setup() {
  Serial.begin(115200);
}

void send_mavlink(mavlink_message_t *mavlink_message) {
  uint8_t mavlink_message_buffer[MAVLINK_MAX_PACKET_LEN];
  uint16_t mavlink_message_length = mavlink_msg_to_send_buffer(mavlink_message_buffer, mavlink_message);

  Serial.write(mavlink_message_buffer, mavlink_message_length);
}

void send_heartbeat() {
  mavlink_message_t mvl_tx_message;

  mavlink_msg_heartbeat_pack(1, MAV_COMP_ID_AUTOPILOT1, &mvl_tx_message, MAV_TYPE_QUADROTOR, MAV_AUTOPILOT_GENERIC, MAV_MODE_FLAG_MANUAL_INPUT_ENABLED, 0, MAV_STATE_STANDBY);

  send_mavlink(&mvl_tx_message);
}

void loop() {
  send_heartbeat();
  Serial.print("Plain text \n");

  delay(1000);
}

On my computer, I want to use Pymavlink to read and interpret messages. I have the following code:

from pymavlink import mavutil
import time

conn = mavutil.mavlink_connection(device='COM7', baud=115200, timeout=1)

print('Entering while loop')
while True:
    msg = conn.recv_match(blocking=False)

    if msg is None:
        pass
    elif msg.get_type() != 'BAD_DATA':
        print(msg)
    else:
        print(msg)

    time.sleep(0.001)

However, this does not work. Sometimes I will get no read characters, other times it will print ~8 lines similar to this:

BAD_DATA {Bad prefix, data:['73']}

and then stop printing.

If I try the Serial Monitor of the Arduino IDE, every second I see characters being printed and at the end of the line Plain text, as I would expect. What am I doing wrong that prevents Python from reading serial?