Bug in SAS' SELECT DISTINCT implementation

Running the following code on SAS version 9.2 (TS1M0) on the XP_PRO and NET_SRV platforms, you get erroneous results on the second SELECT DISTINCT. Dataset with_index should only have a single record in it, but on the above-named platforms you get 2 records. It seems that SAS is ignoring the DISTINCT.

  %let startdt  = 01jan2009;
  %let enddt    = 31dec2009;
  
  data enrollment ;
    input
      @1  mrn       $char8.
      @9  enr_start date9.
      @21 enr_end   date9.
    ;
    format
      enr_: mmddyy10. ;
    ;
  datalines ;
  roy     01jan2008   30jun2009
  roy     01jul2009   30jun2010
  ;
  
  proc sql ;
    create table without_index as
    select distinct mrn
    from enrollment as e
    where e.enr_start <= "&enddt"d and
          e.enr_end   >= "&startdt"d
    ;
  quit ;
  
  
  proc sql ;
    ** This guy queers the DISTINCT result ;
    create index pk on enrollment (mrn, enr_start, enr_end) ;
  quit ;
  
  proc sql ;
    create table with_index as
    select distinct mrn
    from enrollment as e
    where e.enr_start <= "&enddt"d and
          e.enr_end   >= "&startdt"d
    ;
  quit ;

I'm not sure exactly what the conditions are for this bug, but the above definitely ain't right.