API queries filtering on contact_sub_type don't work

I have this problem on the Joomla demo site (4.6.0) and my own sites with versions 4.6.8 and 4.7.1.

If I use the API explorer or run test php code, with a query checking for multiple contact_sub_type, I get 0 results:

$result = civicrm_api3('Contact', 'get', array(
  'sequential' => 1,
  'return' => "contact_sub_type",
  'contact_sub_type' => array('IN' => array("Student", "Parent")),
));

But if I search using the = operator for one contact_sub_type at a time, it returns the expected results:

$result = civicrm_api3('Contact', 'get', array(
  'sequential' => 1,
  'return' => "contact_sub_type",
  'contact_sub_type' => "Parent",
));

{"is_error":0,"version":3,"count":1,"id":7,"values":[

]}

This happens with subtypes of both Organization and Individual types.

I could work around it by performing multiple api calls and merging the results, but this seems like a bug?

---------------------------

Further experimentation with 4.7.7 on dmaster.demo.civicrm.org gives weird results.

I modified one contact to set sub_type as Student, and one contact to set as Parent.

Using API Explorer, I get these results:

    $result = civicrm_api3('Contact', 'get', array(
      'sequential' => 1,
      'contact_sub_type' => array('IN' => array("Student", "Parent")),
    ));

returns one record - the Parent.

Running a query with contact_sub_type set to equal either Parent or Student returns the expected record.

Running a query with contact_sub_type in an array of either Parent or Student, such as

'contact_sub_type' => array('IN' => array("Parent")),

or

'contact_sub_type' => array('IN' => array("Student")),

returns 25 records (presumably matching a lot more, and just being limited to returning 25.)