Sounds like the vent design is bad. The plumbing should go up several inches before turning toward the side vent. Several feet would be better. If the vent plumbing is too horizontal, rolling would be enough to spill fuel thru the vent.
It could be the fuel return is plumbed into the vent. I don't know what engines you have but they probably don't return enough fuel to agitate the tank.
My vents are 5-6' above the tank tops.
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