Why does wordpress keep adding postname to my CPT’s permalink in the end?

Note that you should use %<post type>% in the slug value, so if the post type is test_cpt, then you would use %test_cpt% as in /a/%test_cpt%/b/c.

And moving the post name/slug to the middle or beginning in the permalink structure is actually pretty easy:

1. Register the post type:

   // replace test_cpt with the correct post type
   register_post_type( 'test_cpt', $args );
   

2. Then after that, do this:

   global $wp_rewrite;
   // replace test_cpt with the correct post type
   $wp_rewrite->extra_permastructs['test_cpt']['struct'] = '/a/%test_cpt%/b/c';
   

And remember to flush the rewrite rules which can be done just by visiting the permalink settings page.