wp_reset_postdata()
restores the post from main query, which you do not seem to be using here at all. So before final the_title()
call you jump out all the way to that post.
Your code seems to be tad problematic to me because both your outer and inner loops continuously rewrite $post
global. If you move away from that in inner loops (by using versions of functions that take post/id to act on) you will likely escape multiple overrides/resets and issues like that.
Something like this:
while ( $my_query->have_posts() ) : $my_query->the_post();
foreach ( $post->actors as $actor ) :
echo get_the_title( $actor );
endforeach;
endwhile;