Please help: what is wrong with my ceiling?

[fbg002]

I have a room with sloped ceilings and various wall heights as shown. I drew the room as a rectangular room, and split the side wall twice to make the slopes. The walls form a contiguous rectangle, with heights that match piece-by-piece. I would expect "ceiling height from nearest wall" to work fine, but please see the image. It does not. What is wrong?

Thanks in advance for any help. Image at: https://imgur.com/a/nn1w0fm

[Puybaret]

Unfortunately, the algorithm that computes ceilings doesn’t work so well.
But as the shape of your room is not so complicated, it should work if you draw two rooms instead of one : a room containing the top part and one sloping part, beside a second room for the other sloping part.