Assignments Date Track History disappeared

Hi,

We have one employee who had 9 records of date track history now has only 3 records below, any idea what scenario this can happen, we are not sure what the user has done to get it into this scenario.

earlier

----------

ASSIGNMENT_ID    EFFECTIVE_START_DATE    EFFECTIVE_END_DATE
848    01/05/1996    24/10/2007
848    25/10/2007    25/10/2007
848    26/10/2007    03/09/2008
848    04/09/2008    01/10/2008
848    02/10/2008    01/03/2010
848    02/03/2010    22/03/2010
848    23/03/2010    19/08/2010
848    20/08/2010    22/05/2011
848    23/05/2011    31/12/4712

Now

-----------

ASSIGNMENT_ID    EFFECTIVE_START_DATE    EFFECTIVE_END_DATE
848    01/05/1996    31/12/2006
848    01/01/2007    28/06/2011
848    29/06/2011    31/12/4712

Rgds,

-Kamal