I tested this and I was able to reproduce it. Whenever you use the room tool, it wipes the wall paint on both sides of the walls. I was able to do it with the wall tool too if I tried to delete an exterior wall section. The wall section didn't delete but the paint did.
On apartment exteriors, it is hard or nearly impossible to select the outside wall paint with the eyedropper tool but if you go to the wall paint section and select any paint and try to apply it to the anywhere on the exterior, the entire exterior finish gets replaced with what was there originally. The game will not let you change the exterior finish, it just restores what was there.
So, imo, this is just a quirk of the build system.
Hope this helps.
