Solved!
the magic was Serial1.readBytesUntil() function if i define it correctly.
reason for missing charachters was Serial (not Serial1 which passed data from server) doing stuff adjacent to serial1 in background.
serial is very realtime function thats sensitive to background "noises".
solution.
muting/waiting serial while serial1 passes data.
passing data from serial1 to an array rather then writing or printing to console.
after that - i handles the data received.
thats the core of the solution:-
char serialdata[100];
Serial1.readBytesUntil("Z", serialdata,100);}
int _count;
String _result ="";
while (_count< 100){
_result +=String( serialdata[_count]);
_count = _count+1;}
Serial.println("result"+_result);
hope i contributed something as a novice here.
thanks.