when query a dimension having null value with not equal or not in get wrong result

[lk_hadoop]

hi,all:
      when I query with this sql : 


      SELECT NATION, DATES, IS_JOIN ,SUM(REC_AMT) AS sum_REC_AMT  FROM 
GJST.SH_FETCH_SALE_BASE_FACT_ALL_NEW
      WHERE NATION IN ('控股') AND DATES IN ('2020-02-11') GROUP BY NATION, 
DATES, IS_JOIN;
     
      I can see there is a null value in IS_JOIN column.


           


     but when I query with :
     SELECT NATION, DATES, IS_JOIN ,SUM(REC_AMT) AS sum_REC_AMT FROM 
GJST.SH_FETCH_SALE_BASE_FACT_ALL_NEW
     WHERE NATION IN ('控股')
     AND DATES IN ('2020-02-11')
     AND IS_JOIN <> '加盟'
     GROUP BY NATION, DATES, IS_JOIN;


     the null value is not in the result set:
     
    is this a bug ? I'm using apache-kylin-2.6.3-bin-cdh57