I haven't had any problems with the residential rentals that I've build. But I did build mine a little differently in that I did not change the venue type until after I had completed the build, then set the venue to residential rental and assigned the spaces to the apartments. Both buildings I've done had 6 apartments. So, maybe that has something to do with causing your problem since you set the lot as residential rental first. Just a guess though.
But here's something you can try to get the lot fixed. First thing, run a game repair and delete the localthumbcache file. Start the game and go to the lot in build mode. Undo all the space assignments. Set the lot to residential. Save the lot to your library. Bulldoze the lot (don't just load the copy on top of the existing build). Replace the lot with the copy from your Library. Then enter the lot from the map. Change the venue to residential rental. Assign the spaces to the apartments.
Hope this helps.
