HERE'S A GOOD IDEA I KNOW I'M A BIT LATE IN THE GAME BUT.... CHECK THAT YOUR TANK BAG ISN'T BLOCKING THE PETROL TANKS AIR INLET ON THE FILLER COVER THEREFORE CREATING A VACUM IN THE TANK NOT IN THE FILTERS OR A BIT OF DIRT BLOCKING THE SAID HOLE HEY, GIVE IT A GO YOU NEVER KNOW

I am in Mongolia and am having this exact problem. Did you ever find the cause or it just fixed itself?