Arduino Nano Every Serial Problem

Hi,
I made a custom libary for the Arduino Nano board, where it worked perfectly, but something was wrong with the new Arduino Nano Every board.
The purpose of the program is read bytes (from a display) on serial port, and then run the program based on it.
It cannot always read the data. Changing Serial.setTimeout() improve slightly but still not right.
Has anyone experienced a similar problem? If it hadn't worked on the old Nano, I wouldn't have asked here, but it was good there and I could not found enought information about Every that would move me forward. I changed "Serial" to "Serial1", I didn't change anything else.
Essential part of the code that is skipped even when bytes are coming in:

if(Serial1.available()>0) 
{
     
    Serial1.readBytes(byteRead, 6);

}

In the header file: static byte byteRead[6];
Thank you in advance for your help!

Do you have a minimal sketch that demonstrates the problem, or is this only tested in some large sketch of yours?