problem using Software Serial

since i wanted to compare the analog value, i did the comparison on the TX board, and sent a char over mySerial('f' for false or 't' for true) to the RX board, there on the RX i stored that char, then used switch\case to set a boolean flag for the 'if' statements..
it worked fine!
tx:

if(value<450)
  {
    mySerial.write('f'); 
    
  }
  if(value >900) 
  {
    mySerial.write('t'); 
    
  }

rx:

char RXdata; boolean flag=false;
void loop()
{
   
  mySerial.listen();
  if (mySerial.available()>0)
  {
    RXdata = mySerial.read();
    switch(RXdata)
    {
      case 't':
                flag=true;
                break;
      case 'f':
                //flag = false;
                break;
   }
  }
  if(flag)  
  {// do something}
  else{//do something else}
}