Hello Bookman,
Would you mind showing me the post processing settings of the actor you are applying motion blur? If the effect is being applied through a ‘Post Process Volume’ then make sure it is set to unbound and you are not having its priority overridden by your camera actor within the ‘Matinee’. The motion blur seems to be functioning correctly on my end, so check the places I mentioned and see if you may be overlooking something.
Cheers,