December 2017 - last edited February 2018
Product: The Sims 4
Which language are you playing the game in? English
How often does the bug occur? Every time (100%)
What expansions, game packs, and stuff packs do you have installed? All
Steps: How can we find the bug ourselves? 1) Draw a room at ground level, 2) draw a basement room beneath it, 3) raise foundation height to greater than one notch up, 4) add stairs from the basement up to the room above it. You may need to rotate the lot in order to see the effect, as the sun's position does seem to factor into it. Note that you should *not* add any interior lighting or windows/doors, so there is no confusion about any light you do see in the interior.
What happens when the bug occurs? Exterior light is visible on a portion of the stairs, when viewed from the basement, and from the room above, you can see a the white seam of the top of the basement wall where it meets the foundation. If you change the time of day, you will see the color of the top of the wall change accordingly, showing that it is exposed to exterior light.
What do you expect to see? If the structure is fully enclosed (as in the example I have described), I would expect to see no exterior light entering the basement.
Have you installed any customization with the game, e.g. Custom Content or Mods? Never used.
Did this issue appear after a specific patch or change you made to your system? No
This screen shot shows the light on the stairs as viewed from the basement:
This screen shot shows the top of the basement wall where it meets the foundation - it appears bright because it is receiving external light: