Hi Martin, thanks for posting the file. Yup, that's a bug - the bug is that it looks at segments and if a segment is on a parallel plane with respect to the revolve axis it thinks it does not need the additional processing. The bug is that it should be checking this planarity across the whole path curve and not just on individual segments.

It should not be difficult to fix for the next beta release. In the meantime, since the problem only happens on things that are planar but at a different level, it's pretty easy to fix them - use Edit > Separate to break it apart into individual surfaces, select the too tall surfaces, go to the Front view and click one time on the edit frame handle to switch it into "Scale from opposite corner" mode instead of the default "Scale from center" mode, then grab the bottom left edit frame handle and drag it upwards until it snaps onto the rail curve.

Thanks - Michael