Happens with any other mount too, existed very long. Its an engine thing, the server cant differentiate between a stopped jump (by the solid bridge) or a real jump sometimes and if you spam jumps it will put you on the z coordinate it expects you to be after jumping. I fear they cant really do something about it, it is not game breaking either and happens at a lot of other locations too. They could make some invisible walls under the bridge so you cant get so close under it or make the bridge thicker but they never really touch old maps so whatever...