AFAIK, the current rendering is intentional: by rendering platforms and railways on top, you can actually see the layout of the railway station, which may be useful for micro-level orientation. With buildings rendered on top, you wouldn’t even know for sure which tracks actually run through the station, or actually end inside the complex, like with a head station.
Not necessarily. The layer key describes the vertical ordering of things in the real world. This information is available to renderers, but whether and how they make use of it is up to the designers of the render style.
The default map style happens to render things by a defined priority, rather than according to real life top-down order. This allows “important” things such as roads to be visible even in places where they are covered by buildings or similar features. So this is likely not an error, but simply works as intended by the map style’s designers. Opinions may vary on whether this design decision was/is a good idea, of course.