Steps to reproduce:

-Set up a valid structure save area using corner blocks.
-Teleport yourself approx. 50 - 80 blocks away, starting from one of the corner blocks.
-Place a save block, use the same name (obviously), and click detect.

It'll detect the structure, but sets up an invalid savebox. The invalid part can manifest itself in multiple ways, but in all cases the save box won't involve the intended area.

Included screenshots show me standing on one of the corner blocks (to share coordinates), and the save block and its coordinates as well as the invalid set up savebox. Further in the background you can see the house (intended save area).

I've also added a 3rd picture which shows the intended savebox, so that you can rule out the option of me using an invalid sized area.