Yesterday I befriended a few apes, and this morning when I opened my game, two had apparently crossed the ocean to my home island to see me. They started out stuck on the shore right where the water meets the sand and would appear nearby whenever I traveled to a new island, but they've found a way to get onto dry land.
It's not as bad as it could be since they aren't stealing all my stuff, but I did choose my island specifically to be away from prime apes.
Steps to Reproduce
Befriend apes on distant island, play for several more hours, save and quit, come back the next morning and find apes have appeared at home island.
This issue has not been confirmed by a developer yet.