I noticed that you haven’t connected your custom post type and taxonomy completely. I’m not sure if this is causing your current issue, but it would be the first thing I would try.
From the Codex:
When registering a post type, always register your taxonomies using
the taxonomies argument. If you do not, the taxonomies and post type
will not be recognized as connected when using filters such as
parse_query or pre_get_posts. This can lead to unexpected results and
failures.
So, I would add to your post-type args:
'taxonomies' => array('merk'),
and see if that fixes your problem.