No, I did not taxi backwards. Ha Ha! It is entirely possible that this was a fluke. My left engine was overheating everytime I got to the runway before the last update - it does not overheat now. I did notice that although it was smoking and running rough it did not register as a failure when I ended the mission.
On the occasion I mention above I thought I had shut down the left engine and I was sure I was at a full stop at the seaward end of Manston airfield. Due to the bug I thought my engine was overheating as it was smoking and rough. I stopped the left engine but I still had both engines selected as I increased the throttle input and began to taxi. So there may have been a difference between the physics and the graphics.
Either way, hard to test as the new update has changed the conditions. Left engine works fine now....
Edit: I went back and attempted to replicate the situation (albeit with the new update) and, although I was able to get a bit of forward momentum it was not enough to actually taxi anywhere - I just wound up going in large circles until my right engine burned out. Must have been a fluke before.