Mine has leaked with full tank, half tank or almost empty tank.
Dealer replaced the check valve under warranty supposedly with a newly designed one from Kawi. The new one is better, but still leaks.
Dealer has checked with Kawi and the tube is not supposed to be leaking gas at all, that's what the check valve is supposed to stop, the gas from coming out.
Purchased my bike in January and the first valve leaked gas even on cold winter days.
Going back to the dealer with this, I'll update accordingly.