Consider yourself lucky. I think it is definitely that check valve at the tank sticking. Probably because of the ethanol additive in the gas affecting the rubberized valve. Lot of people experiencing it but for sure not everyone. Even saw where one person identified that it was stuck open when he removed the tank. He hit it and forced it closed again. Another person said his always burped gas back until he was in an accident and got hit right in front of the gas fill. When he got his rig back from the body shop it hasn't done it since. Either they replaced part of the filler or the impact knocked it loose. Tough way to get it fixed!
Mark
Mark