Low Blow, check your range to target when the missile impacts. I was testing this issue yesterday and got a kill from 12nm as well, but by the time of impact I had just crossed over the 8nm range so didn't see the effect of the bug.
If you turn away from the target after launch, say 45 degrees you last bit of the missile flight will be a sharp turn toward you while the maverick tries to chase down the dot at the 8nm mark.