Hopefully some will find the following useful, or at least food for further discussion.
I had a look at the first 80 starter kit style maps. I did not look at any double-wide boards, original hardback boards, deluxe, or TPP boards.
Using the literal interpretation of B20.1, my non-scientific conclusions follow:
Board 13 - I had no problem determining what the stream end hexes are.
Board 22 - K1 is questionable, but I would argue W10 has just enough of the stream art going to a second hexside to declare it a stream hex.
Board 32 - No problem determining stream end hexes.
Board 34 - Ditto.
Board 36 - K10 is questionable, and Q5 has 3 hexsides, putting it in conflict with a pedantic reading of B20.1. Otherwise OK.
Board 40 - No problem.
Board 47 - Yikes. All end hexes are questionable. Also AA6 and G7 have 3 hexsides.
Board 50 - Questionable (as per the OP).
Board 66 - No problem.
Board 72 - No problem. The stream art is really well done on this map, following the dictates of B20.1, and making it very apparent (to me) what was intended as a stream hex and where said stream ended.
Board 76 - Ditto as board 72, but AA8 has 3 hexsides.
YMMV