Filtering using a related column only displays a maximum of 100 records.
Toolset has added the limit option in the toolset_get_related_posts function, with a default value of 100.
Adding the limit=-1 option causes the filter to display all results.

$posts = toolset_get_related_posts(
  $value->get_value(),
  $this->relationship,
  [
    'query_by_role' => $this->role,
    'limit' => -1,
    'role_to_return' => $this->return_role
  ]
);