Okay heres what happened. You invited rosie from my town but before she could place her plot down, you invited diana. Correct? If so, then diana replaced rosie as a move in so yeah.. You lost rosie.
Yu should always make sure a villager placed her plot down before you invite neone else into your town from campsites or from other traders. Or else the 2nd invite will replaced the first... In this case... Diana replaced rosie.