I'm pretty sure this is unintentional. I mean, wagon's are stopped by an odd rock, and decide to bypass because one such rock happens to be in their way, but not a physical support, which holds itself up and doesn't need to be adjacent to a wall?
Wagons have always been stopped by rocks (in the 2D Version you had to build a road outside, then smooth the inside in order to have caravan access), but not supports. If it is unintentional, it's been around a long time.
So have a lot of things.
The issue is that things are either passable or they're not, for the most part. A wagon can pass through furniture just like a dwarf can.
The issue is that this makes sense for single-tile creatures, but not multi-tile ones. A single-tile creatures can presumably walk through supports/cabinets/whatevers because they don't take up the entire tile. However, this doesn't make a hell of a lot of sense when
you take up more than an entire tile. In this sense, wagons shouldn't be able to pass through things like furniture either, but that creates a lot problems and issues, especially since they have to park in a trade depot full of such things.
So it's not really a "bug", per se, just a quirk of how the engine currently handles things.
Perhaps this kind of support?
That doesn't really make any sense, though. The supports in DF aren't along the walls, they can exist in open space.