These are the "3D" bridges in which an invisible platform is raised to cross when going over the bridge and the platform drops to the ground when going under. There are a couple of bridges in Plutonia 2 where you can fall through them if you are standing on them. You're fine if you walk or run over the bridge. If you fall through, you will be trapped within that platform that rises/falls, usually a rectangular shape. This doesn't appear to happen in other ports.
There are at least two areas where this happens. One of them is the bridge in Plutonia 2 Map05, right behind the starting area. There is one at -885, -665, -272 in Map11 where this issue also happens.