It sounds like it could be this issue. @josephdevon007 did you build the new room in the basement, connected to the original room, or were you building above ground? Did you have bb.moveobjects turned on or off? When you say the room became undesignated as a room, what do you mean? Were there missing walls or flooring?
It does sound like a similar issue. It was in the basement so the floor disappeared along with the stairs.
However there were numerous sims on the lot that I was playing but all of the items have disappeared. They are not in any inventories. I'm most interested in recovering all of the lost collectibles.
Did you ever make a backup ?
Did you check the auto-backups to see if they still contain them:
http://sims4.crinrict.com/en/2014/09/save-games/#wiederherstellen
If they do, you could try uploading those houses to the gallery and re-placing them in your current game.
