I did a game repair last night and that gate was still closed. On the first trip to Selvadorada, only the gates to the airplane, the lagoon, royal bath, and temple were open.
On the second trip to Selvadorada, only the gates to the royal bath, Omiscan Garden, the lagoon, and the temple was open.
I have sent two sims there for vacation from two different families, and the island gate closed on their first trip down there.
I have started a new game with a new family because I had ran into some glitches while playing yesterday. So now I'm trying to see if these glitches return with this family. But I do plan to send the current family down there to see if they can reach the island.