The materialized view was in good status until when changes its refresh mode:
SQL> select MVIEW_NAME,REFRESH_MODE,REFRESH_METHOD,STALENESS
dba_mviews
where mview_name='MVIEW1';
MVIEW_NAME REFRES REFRESH_ STALENESS
-------------- ---------- --------- ----------
MVIEW1 COMMIT FAST FRESH
SQL> alter materialized view MVIEW1 refresh fast on demand;
SQL> select MVIEW_NAME,REFRESH_MODE,REFRESH_METHOD,STALENESS
dba_mviews
where mview_name='MVIEW1';
MVIEW_NAME REFRES REFRESH_ STALENESS
-------------- ---------- --------- ------------------
MVIEW1 COMMIT FAST COMPILATION_ERROR
SQL> alter materialized view MVIEW1 compile;
Materialized view altered.
SQL> select MVIEW_NAME,REFRESH_MODE,REFRESH_METHOD,STALENESS
dba_mviews
where mview_name='MVIEW1';
MVIEW_NAME REFRES REFRESH_ STALENESS
-------------- ---------- --------- --------------------
MVIEW1 COMMIT FAST UNUSABLE
It might be a bug ? not quite sure why. In this situation, the materialized view has to be refreshed completed or rebuilt.