Things I can think of:
1. What you think is the vent hose really isn't and there's a plug in the real one.
2. There's both a disconnect in the vent hose AND a plug, with the plug being on the tank side and the disconnect beyond the plug.
3. The vent hose connection is on a part of the tank that also gets lifted up as it fills, so the water level isn't making it to the overflow point.
4. A more likely cause, given the circuitous vent hose path you describe, is that the hose travels too high relative to the top of the tank before descending. Since that overflow hose is not fed by pressure but by gravity, you wouldn't see flow until the top of the overfilled tank got above the top of the vent hose. Then it would drain and probably siphon itself down to the normal water level. That's what I would check first.