friend! I noticed the same problem, only on the PC I am writing a program in Delphi. Everything seems to be correct, I tried adding additional time delays and buffer cleanup to different places in the PC and arduino programs, but I still haven't figured out what the problem is. Everything is exactly the same as you wrote - after turning on arduino, the first package from the program on the PC does not reach it, but if you restart the program on the PC, the first package begins to reach it. The only solution I found was to send the first package from the PC twice.