Slitherine

Please look at the screenshot, at the Artillery unit marked with a broken white circle.
The screenshot is taken from the map Gijon, AO Spanish Civil War.
Left and right you see a large river. Large rivers are detrimental to supplies. So my question is why is this unit not marked as encircled?

The edge of the map works as a supply source. Since there is nothing that blocks the path to the edge, the unit is in supply.

Its been that way all along it's just sometimes its hard to delineate the boundaries