This Boeing 747 of Air Atlanta operated a flight between Düsseldorf and Jakarta (Indonesia / 6100 miles). The captain of this flight saw my video and responded: “It”s actually fuel coming from vent line via the wing tip vent. I was the capt on the flight and had to fill 600kg of paperwork afterwards”. Another engineer told me that there”s a wing fuel tank vent box in the tip of all planes. When the fuel gets too full in the tanks it overflows into the vent box and (obviously) out the vent. The wings flexing probably caused the fuel to shift around!