Could be that you are mixing up which portal in cape that is an old exported map and one that is the actual map that is being worked on in the game.
Keep in mind that a map in the game folders being worked on that has "Has portal" checked will place a portal in cape, if you also export that same map without removing it from the exports folder afterwards it will ALSO create a portal in cape leading to the very same mapname, the difference? One portal will lead to an old or new export, and the other will lead to the map the map maker uses, so do make sure there's no extra copies of your map laying around in the exports folder.
If it wouldn't be what was mentioned above, it feels very likely you're either saving it wrong, or not placing the destination coordinates correctly causing it to go elsewhere. Check the video below if you need to double check things are right.
https://www.youtube.com/watch?v=9cGLY79x4yg