TOPIC:

This is due to the extended walls, and is confusing the automatic ceiling creator.
To resolve, select the Projects Tab, and right click on the Ground floor (or the relevant floor) . On the activated context menu, select Properties. In the General section disable Create automatic ceiling. The taper creating by the ceiling generator will disappear. If you require a ceiling in this partial plan, then you must draw the ceiling manually.

Applicable to all versions

I have created a partial plan to show an individual room. Although the wall appears correct in the 2D plan, the 3D view shows a taper in the walls.