Cantilevers and Trusses are different buildings, and treat beams differently. Beams can be tricky to begin with, but aren't bugged.
You can place them:
- On top of the ground (the height value on the object panel screen should match the height value on the terraforming panel)
- On top of cantilevers (cantilever height is 4m, so the height value on the object panel should be [the height value of cantilever]+4)
- INSIDE the top of trusses (the height of a truss is 4m, and the top of it is 0.4m thick, so the height value on the object panel should be [the height value of truss]+3.6)
It's worth noting that on both cantilevers and trusses you can also place 0.1m lower. On the ground you can place them as low as you want, as long as the center isn't 0.4 underground and both ends are supported.
Putting beams on cantilevers and trusses will obviously give you different results. Putting beams on cantilevers for example will let you build a floor out of beams, which will not look right on trusses. Each method has it's own pros and cons.
It would be nice to be able to do everything with both cantilevers and trusses, though.
TL;DR: Neither the beams, trusses nor cantilevers are particularly bugged - trusses and cantilevers currently work in different ways with pros and cons in each way, though I don't know if that's intended or not.
Ground: Height+0
Truss: Height+3.6
Cantilever: Height+4