All of your suggestions was very helpful. but some of my limitation i do not want to use particle CLI. so i don't know
Try out particle serial monitor --follow which should be able to deal with that situation.
will work or not. right now for me Putty is working good. what i made changes. i also used Serial1 which is on PA9 PA10 pins. whose flow conntrol pins are on same USB pins. it could be interfering so i used UART1 with None flow control. and its working good.