1) I know if we take 3.3 V we will get the maxm range to cover from 350ppm onwards(on higher side). but my point was when in specification sheet it's mentioned that at 350ppm it should be within 220-490mV, and if we set it at 3.3V whether it may damage the sensor or not.
2) No i have not tried at any other concentration, not having that facility with me currently.
Is the sensor behaving abnormally?
What I suggest you do is to take your sensor to an open area (outside). and take a few results there, and observe the output. Once you get a stable output (Within the sensitivity levels) then you can make that your CO2 at 350 ppm. Then as libelium-dev said, amplify your output until you get 3.29999 Volts, and from there on, you know that if you expose your sensor to high concentrations of CO2 then your output will decrease.