Could be a stream, but since it only crosses a single hexside it's just artwork. May not be what was intended but B20.1 pretty specifically addresses area like these (though those are among the largest "remnants" I've seen).
From B20.1, "13G4 and I6 are not stream hexes because the stream symbol crosses only one hexside therein. Non-stream remnants such as those in 22J0 and X10 do not block or impede Bypass movement of the J0-K1 hexside."
If a stream, the important thing would be to issue an errata to clarify the issue indicating which other hexside would be touched by the stream depiction. It may have an impact on eligible crest positions.
This should not be too much of a handicap for the product.
I played one of the scenarios with one the hexes. We decided to play it as a stream, because was pretty clear it was intended to be. We laughed, recalled the KGP maps, had more beer. Rolled more dice.