and at arduino is a simple
No, that is not the code on the Arduino. Try again.
Then, you need to understand how serial data transmission works. The data is sent one byte at a time. Slowly.
Your PC app reads all the serial data that has arrived, and assumes that that is a complete packet. It is not. When you consider ways to delimit the packets being sent, and read and store data until a complete packet arrives, your problems will be solved.
How to do that only comes up about 3 times a week on the forum, so it might take you a few seconds to find some solutions.