There are several islands in the game that are not closed upon completion of the main quest or when moving to the next map.

Balancer Island, Roaring Island, Sapphire Island.

These islands are the last in their storyline, there is no continuation yet, so the islands will remain open for now. You can temporarily use them as an additional source of resources. They do not affect the subsequent islands.

Mysterious Island

This island usually closes after the player moves to Invention Island. If Invention Island has already been completed, but Mysterious island is still open, although you finished all the tasks, you can contact the support service.

This island closes after players travelled to Iron island at level 45.