River/Isolation/Engineer relief problem

[Grrr]

The odd thing is, in a similar situation, playing the same scenario crossing the same river, a few hexes North, I ferried across a force using a engineer unit. The ferried units were in supply so long as they stayed within one hex of the river, As I understand the rule is supposed to work. So, I'm still confused why their is a difference in my current game. I wonder what is different?