Territories on a fully tiled image can wrap around, thus not constrained to the image border. Maybe this was never tested(?).

The corner territory will appear at the four corners of the map, and is divided into four 'islands', one of which is not drawing. Other border territories also have display anomalies. The territory label and value get hidden by one region but not the mating region, thus partially hidden (sliced). Same with units and their numbers. In the case shown, the entire territory label is missing. There is only one unit, but if there were more, then for this placement the number would be partially hidden.

It looks like display order is the culprit in all cases.

The small-map in top-right corner is a good example of graphically challenged all along right and bottom edge.

It also shows a unit getting vertically sliced along with the territory label beneath at the map border.

These engine errors can be tested with high repetition rates using CRETACEOUS. New map available here:

    https://john.gregorek.com/Games/cretaceous.zip