083.99549 W
OK, I know what you mean then.
84 deg is a UTM Zone boundary. This is the only place you will see map edges on TerraServer tiles. Most maps you download won't run into this, but you are assured of running into this fro a long thru huking trail. The CDT crossed the boundary 6 or 7 times.
BackCountry Navigator tries to do the best it can there. It will render tiles that are in the same UTM Zone first, then reproject and try to find any tiles in the adjacent UTM Zone.
I wish TerraServer didn't have all those map edges scanned. I don't what good they think they are.
There is not an easy way to get around them, unless I add a way for you to delete tiles. Even then, you could only delete full tiles, but it does look as if there are full tiles of junk there.
When you are actually navigating, this won't be as big of a deal. It will look like you are about to go off the edge of the map and then pop, you jumped to the next zone.
How soon does your AT hike begin? That will tell me if I have time.
And how are the maps fitting? The CDT is about 887 Megabytes, but that was with some precision downloads.