I was having a similar problem, but in my case, there was no motion blur at all in the rendered movie.
I found out motion blur would be applied to the final render when I played the game in editor once. Also, I don’t know if it actually makes a difference, but I’ve set my sequence to Auto Play for it to play once in PIE too. So just hit play, wait for it to start playing and stop.
Perhaps PIE creates some required buffers or something that the Movie Render feature requires.