After completing the final mission on the island and moving to the next one in the storyline, a window with a warning will appear. It will inform you that moving to the next island will close the current one in 24 hours.
If there are still many resources on the current island that you would like to take home or if you have unexplored territories, just ignore the window and tap on the button "Stay".
🔥 Do not move to a new island until you are sure that you have done your best on the current island!
💡 It is also worth noting that there are several exceptions to this rule, which you can find in the article at the link.