The readyRead() signal of QSerialPort does not triggered correctly.

I used the QSerialPort in my project and the slot function would be triggered by the signal readyRead(). Begin, the version of my Qt IDE is 5.13.2. When I tested my program, the serial port didn't work well. In a loopback test, when I sent the first line of my data, it didn't receive any data. However, when I sent the second line of my data, the first line of data would be received simultaneously. And when sending the third, the second will be received, and so on. To my puzzle, I looked over my code and confirm there was no problem in my code. The 5.13.1 and 5.13.3 have the same problem. So I rollback to 5.12.6. The problem looks like being solved. But unfortunately, after working for a little while, the same problem occurred in the program based on 5.12.6, while other serial tools worked well. Additionally, the examples provided by Qt also did not work well. And I searched on the Internet, some people encountered the same problem.