ACF order relationship by date

The $journalEntries IDs are a post_type = post? Because if you don’t specify the post_type in the WP_Query it will default to ‘post’.

So your $args array should be:

$args = array(
    'post_type' => 'put_post_type_here',
    'post__in' => $journalEntries,
    'orderby' => 'date',
    'order' => 'DESC',
);