It's silly, stupid, whatever.
The airbox. It has a foam band around it, whitch touches gas tank from the inside.
It looked like a factory one and in place until I tried and looked through filterbox from another TRX.
It was the foam. It's too solid and when you put a tank over it, it blocks air supply to the upper part of filter box, where the intake hole is.
Removed it and everything is fine now.
By the way, it looked really factory installed. Faulty assembly? Mistake? Who knows.
Thanks all for response.