It's not a bug, it was intentionally designed like this, because there are many custom ground vehicles, which are also much more frame rate friendly than the default ones so, the only way to be sure they do not appear, is to flag the parking spots as Ramp GA, because otherwise users would be forced to set aiport ground traffic to 0 each time they entered KJFK and, those that didn't, would see double objects and very low fps.