Can't map X-MICROSOFT-CDO-BUSYSTATUS

Thanks for this great module, I love it.
There is only one thing I can't get to work and that is mapping a custom X-Property like X-MICROSOFT-CDO-BUSYSTATUS
I can map any other custom property like STATUS or PRIORITY with the hook_date_ical_import_timezone_alter() function but when I map a custom X-property like X-MICROSOFT-CDO-BUSYSTATUS:BUSY not the value BUSY is pushed in the Drupal field but the string "X-MICROSOFT-CDO-BUSYSTATUS" (see screenshot)

Here is my custom module where I successfully map STATUS but where I can't map X-MICROSOFT-CDO-BUSYSTATUS

/**
 * Implements hook_date_ical_import_timezone_alter().
 */

function date_ics_mapping_date_ical_mapping_sources_alter(&$sources) {
  // Add the "STATUS" ics Outlook property to the mapping sources.
  $sources['STATUS'] = array(
    'name' => t('Status'),
    'description' => t('The status of the appointment. It is CONFIRMED .'),
    'date_ical_parse_handler' => 'parseTextProperty',
  );
  
  // Add the "X-MICROSOFT-CDO-BUSYSTATUS" ics Outlook property to the mapping sources.
  $sources['X-MICROSOFT-CDO-BUSYSTATUS'] = array(
    'name' => t('MicroSoft Busy Status'),
    'description' => t('The X-MicroSoft Busy status of the appointment. It is BUSY or TENTATIVE. Tentative is the state before accepting the anointment by the delegated'),
    'date_ical_parse_handler' => 'parseTextProperty',
  );
  // Add the "PRIORITY" ics property to the mapping sources.
  $sources['PRIORITY'] = array(
    'name' => t('Priority'),
    'description' => t('The Priority status'),
    'date_ical_parse_handler' => 'parseTextProperty',
  );
}

The X-MICROSOFT-CDO-BUSYSTATUS comes from a Exchange ics file. Here is a part of that:

BEGIN:VEVENT
SUMMARY:My test appointment 50 
DTSTART;TZID=W. Europe Standard Time:20160422T070000
DTEND;TZID=W. Europe Standard Time:20160422T110000
UID:040000008200E00074C5B7101A82E0080000000020FA569E4B9AD101000000000000000
 0100000007EDD6BCC2144ED4D83B8F6CDC0CFDE97
CLASS:PUBLIC
PRIORITY:5
DTSTAMP:20160419T194020Z
TRANSP:OPAQUE
STATUS:CONFIRMED
SEQUENCE:0
LOCATION:Rotterdam
X-MICROSOFT-CDO-APPT-SEQUENCE:0
X-MICROSOFT-CDO-BUSYSTATUS:BUSY
X-MICROSOFT-CDO-INTENDEDSTATUS:BUSY
X-MICROSOFT-CDO-ALLDAYEVENT:FALSE
X-MICROSOFT-CDO-IMPORTANCE:1
X-MICROSOFT-CDO-INSTTYPE:0
X-MICROSOFT-DISALLOW-COUNTER:FALSE
END:VEVENT
BEGIN:VEVENT
SUMMARY:My test appointment 100
DTSTART;TZID=W. Europe Standard Time:20160422T130000
DTEND;TZID=W. Europe Standard Time:20160422T150000
UID:040000008200E00074C5B7101A82E00800000000201B3A671F9AD101000000000000000
 010000000A9274B22BB5DEF48922A4EA931F7B921
CLASS:PUBLIC
PRIORITY:5
DTSTAMP:20160419T194020Z
TRANSP:OPAQUE
STATUS:CONFIRMED
SEQUENCE:0
LOCATION: Amsterdam
X-MICROSOFT-CDO-APPT-SEQUENCE:0
X-MICROSOFT-CDO-BUSYSTATUS:TENTATIVE
X-MICROSOFT-CDO-INTENDEDSTATUS:BUSY
X-MICROSOFT-CDO-ALLDAYEVENT:FALSE
X-MICROSOFT-CDO-IMPORTANCE:1
X-MICROSOFT-CDO-INSTTYPE:0
X-MICROSOFT-DISALLOW-COUNTER:FALSE
END:VEVENT

I can use some help here:

1. So what do I do wrong?
2. How can I map X-MICROSOFT-CDO-BUSYSTATUS?

Thanks for your time!