First I loaded world (save game) from one slot where I was on the second island. Then I realized i wanted to play the other world and went back to main menu and loaded world from another slot where I was on the first island. After that I did not have any bats attack until I went to second island through the ruins. Right after leaving the ruins on the second island I was attacked by a crazy amount of bats (first screenshot). When I ran away, another smaller bats wave occurred (second screenshot). After going back to first island bats waves occured normally on the first island.
Load one world with player on the second island, exit to main menu, load the other world where player is on the first island.