I know this is an old thread; but it didn't really provide a solution. I had this same problem - I tried painting a wall and just got a horizontal stripe midway up the wall. I got the same results whether I selected the paint from the library and applied it to the wall (using the paint roller), or by opening the Room Specification and changing the wall covering (Room Specification > Wall Covering), or by opening the Wall Specification and changing the wall covering there (Wall Specification > Wall Covering). When doing the last two, I noticed that the Position section had some strange values. The height was 8" and the Top to Ceiling was 50-something and the floor to bottom was 40-something. I cleared all of the values and saved. I am happy to report it worked.
I think the problem might have been a result of increasing the room height (by 8") after I had already built the floor.
Sharing this info in hope that it helps someone.
