Methodology to debug tcdrain() blocking indefinitely - c++

Question

I'm sending packets from an Ubuntu machine to an STM32 autopilot through a USB cable using write() and tcdrain():
int Serial_port::_write_port(char* buff, unsigned int len) {
const std::lock_guard<std::mutex> lock(_mutex_write_on_port);
const int bytes_written = static_cast<int>(write(_fd,buff, len));
if(bytes_written<0) {
printf("write to port failed with errno: %s\n", std::strerror(errno));
}
//wait until all data has been written to the port
tcdrain(_fd);
return bytes_written;
}
After a few thousands packets successfully sent, tcdrain() randomly blocks. I'm trying to understand why.
Any recommendations on how to debug this?
I've done some research but couldn't come across a good debug methodology.
Note: I've noticed if I run a simple python command in another terminal ser = serial.Serial('/dev/ttySAC0') this "unfreezes" my code about 75% of the time but I'm kind of at a loss to understand why.
If that helps, here is how I configure the port:
int Serial_port::_config_port() {
//Config based on following sources:
//https://github.com/mavlink/c_uart_interface_example
//https://blog.mbedded.ninja/programming/operating-systems/linux/linux-serial-ports-using-c-cpp/
struct termios config;
if(tcgetattr(_fd, &config) < 0) {
return -1;
}
// Input flags - Turn off input processing
// convert break to null byte, no CR to NL translation,
// no NL to CR translation, don't mark parity errors or breaks
// no input parity check, don't strip high bit off,
// no XON/XOFF software flow control
config.c_iflag &= ~(IGNBRK | BRKINT | ICRNL | INLCR | PARMRK | INPCK | ISTRIP | IXON);
// Output flags - Turn off output processing
// no CR to NL translation, no NL to CR-NL translation,
// no NL to CR translation, no column 0 CR suppression,
// no Ctrl-D suppression, no fill characters, no case mapping,
// no local output processing
config.c_oflag &= ~(OCRNL | ONLCR | ONLRET | ONOCR | OFILL | OPOST);
// No line processing:
// echo off, echo newline off, canonical mode off,
// extended input processing off, signal chars off
config.c_lflag &= ~(ECHO | ECHONL | ICANON | IEXTEN | ISIG);
// Turn off character processing
// clear current char size mask, no parity checking,
// no output processing, force 8 bit input
config.c_cflag &= ~(CSIZE | PARENB | CSTOPB); //~CSTOPB to set only 1 stop bit for multiflex
config.c_cflag |= CS8;
// One input byte is enough to return from read()
// Inter-character timer off
config.c_cc[VMIN] = 0; //DEBUG: this is different from mavlink recommended setup. Confirm this is ok
config.c_cc[VTIME] = 10;
//baudrate
if(cfsetispeed(&config,_baudrate)<0) return -1; //input baudrate
if(cfsetospeed(&config,_baudrate)<0) return -1; //output baudrate
//write config to port
if(tcsetattr(_fd,TCSAFLUSH,&config) < 0) {
return -1;
}
std::cout<<"port "<<_uart_name<< " configured successfully"<<std::endl;
return 0;
}