Bump
I have the same issue. It seems random. I will be editing footage and all of the sudden the next time i go back to the clip on the timeline, it is squeezed in on the sides. Kind of like a 4/3 image centered in a 16/9 aspect. Premiere is squeezing my 16/9 into 4/3 to make this happen. Other clips around that clip on the timeline do not have the same problem. So i try to delete the clip from the timeline and make a new cut on that clip in the source monitor. Once i import it back to the timeline sequence it retains its correct demensions until I render a new preview, then it switches back to the same problem again. I also have tried to delete all rendered previews, same things happens. All is fine until i render a new preview. I have search all over for a fix to this problem, but have found the answere.