Dunno if this helps, but the Moo boat will do that if I park it with the bow down below the transom. I have to lift it so the bow is higher, even in the garage, with full tank(s). Don't have flip tops, but sounds familiar. My fills are at the front of the fuel tanks which run clear to the transom, I figure it's just gravity, and with heat, expansion, pressure, well, you get the physics. Same thing happens with the Biesemeyer, with flip tops, but it gets stored on the side of the garage and the bow is down a little bit. I'll just never leave more than half a tank in it sitting there,I figure. Seems to be ok that way.
Ray