Well I've noticed it 3 times, the most recent is my current game, but I went back to an autosave to fix the situation. It was laid out like this:
plain city plain
--plain plain
plain hill plain
--plain worker
I told the worker to go to the plain at the #1 position (keypad) to the city, which would have taken the road over the hill. The reason I wanted that is because there was another city off to the left and later I would of course run roads down to it, so by going that way it would save overall road maintenance.
But the worker chose to go to the right of the hill, which would end up making a backwards "C" of roads. Bizarre. The only thing I can figure is the "fastest route" formula.
If a screenshot is essentail then I can recreate the situation, it'll just take a bit of work to do so, thus I'd prefer not to unless critical.