In this vintage video of Heian Shodan (which may or may not show Funakoshi), the 17th move--the third punch in the second pass down the middle--is done as a side punch in horse stance. I've never seen this version of the form before and I wondered if anyone else has. Any bunkai significance?
Besides looking very stiff, I think you are right, that move is being done in horse stance on purpose. If the guy performing it had misstep, I would expect him to be less balanced.
Regarding bunkai, maybe it's a chasing move?
Considering the stances are being executed higher then we generally practice today, that last tsuki could be there as a chasing attack as the adversary tries to retreat from the previous shorter blows.