Serial device skipping every other message (linux termios)

Question

I'm trying to write some bytes to a serial port:

#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>
#include <termios.h>
#include <unistd.h>
#include <stdint.h>

int main()
{
    // Open serial port
    port_fd = open("/dev/ttyUSB0", O_RDWR);
    if (port_fd < 0)
    {
        printf("Error %i while opening serial port: %s\n", errno, strerror(errno));
    }
    
    // Get serial port configuration
    struct termios tty;
    if (tcgetattr(port_fd, &tty) != 0)
    {
        printf("Error %i from tcgetattr: %s\n", errno, strerror(errno));
    }

    // Edit serial port configuration
    tty.c_cflag &= ~PARENB;
    tty.c_cflag &= ~CSTOPB;
    tty.c_cflag |= CS8;
    tty.c_cflag &= ~CRTSCTS;
    tty.c_cflag |= CREAD | CLOCAL;
    tty.c_lflag &= ~ICANON;
    tty.c_lflag &= ~ECHO;
    tty.c_lflag &= ~ECHOE;
    tty.c_lflag &= ~ECHONL;
    tty.c_lflag &= ~ISIG;
    tty.c_iflag &= ~(IXON | IXOFF | IXANY);
    tty.c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP|INLCR|IGNCR|ICRNL);
    tty.c_oflag &= ~OPOST;
    tty.c_oflag &= ~ONLCR;
    tty.c_cc[VTIME] = 10;
    tty.c_cc[VMIN] = 0;
    cfsetispeed(&tty, B115200);
    cfsetospeed(&tty, B115200);

    // Set serial port configuration
    if (tcsetattr(port_fd, TCSANOW, &tty) != 0)
    {
        printf("Error %i from tcsetattr: %s\n", errno, strerror(errno));
    }

    uint8_t testMsg[] = {0xAA, 0xBB, 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x77};
    write(port_fd, testMsg, sizeof(testMsg));
    
    return EXIT_SUCCESS;
}

In this case, the serial device that I am writing to detects the bytes "0xAA" and "0xBB" as the start of a message frame. It then detects the byte "0x77" as the end of a message frame.

For some reason, when I call write() using sizeof(testMsg), the serial device does not respond.

write(port_fd, testMsg, sizeof(testMsg));

However, when I increment the size_t value of the write() call, the serial device responds correctly:

write(port_fd, testMsg, sizeof(testMsg)+1)

Why is this happening? Is there a better way to fix this?

---

for(int i = 0; i < 100; i++)
{
    write(port_fd, testMsg, sizeof(testMsg));
    sleep(10);
}

Actually, when writing the bytes without iterating, the serial device will respond to every other message. Still unsure what is happening though.

---

For reference, the serial device has the following configuration:
Baud Rate: 115200
Data Bits: 8
Parity Bits: None
Stop Bits: 1