I did realise that - and it's a fairly easy fix.
Option 1: Leaving a map switches places with a creep in a position that would "block" a border.
Option 2: Moving from edge -1 to edge moves maps, new units appear on the edge.
Example:
Say you are on the edge at (1, 1) - the act of moving to (0,1) moves you to the next map, and you will appear on (49, 0) on the next map - which makes the edges of the maps unwalkable unless you're coming from a different map, any movement that would result in an edge square moves you to the next map.
Other work arounds that are better then the current system are also available - but that's just my opinion