Apple offline Maps keep deleting themselves

Apple offline Maps keep deleting themselves: I am in South Korea, and flying to Mongolia next week. I had downloaded a map for Ulaanbaatar before arriving here. But, once I arrived, the map deleted itself. I hopped on Wifi, downloaded again, and about 45 minutes later it deleted itself again. I did this several times. The setting to "optimize storage" is not active, and there is plenty of space on the device. Other cities have also been deleted. You can see in the screenshot that the map downloads for use, and then, later, it becomes not available. What is going on?

[Edited by Moderator]