The page template for the grid layout seems to have a bug. It tries to create rows of 4 items but only 3 fit in the width. The problem becomes apparent when there are 8 or more items in the grid.
I made a fix in my child theme. The official Motif theme could be fixed with the following change to the grid loop:
$nr = 0; while ( $child_pages->have_posts() ) : $child_pages->the_post(); echo ( 0 == $nr % 3 ) ? '</div><!-- .grid-row --><div class="grid-row">' : ''; $nr++; get_template_part( 'content', 'grid' ); endwhile; wp_reset_postdata();