Well, I got my first backwards flying dragon, and it's for a bounty quest in Windhelm, and it's unkillable. Got this dragon about 4 hours after I posted I hadn't seen any either... jinxed myself
I can't complete the bounty quest for it so I can't pick up anymore bounties in Windhelm area either.
The effects of the dragon are very erratic. It doesn't always fly backwards, mostly forward, but at extreme speed. If I start to chase it, the dragon clearly is running away from me. Followed it once half way to Riften while killing two more dragons on the way there. It won't let me close to it at all. I have shot it from a distance as it just hovers and took it down to zero health and it still just flaps in the air hovering, will not die. When I do get close to it my game starts to stutter badly. It does attack Once on first encounter then flies up and this behavior takes over. Waited a game-week doing other things and came back to it. I shot it, it hit me with frosty breath and flew up then started this erratic flying again.
Make sure you're looking at the dragon (and the dragon is somewhere you'll be able to access) and open the console, then click on the dragon to target it, then type "kill" (without quotes) and hit Enter. See if that works.