What does the column value 'UNKNOWN' mean in DBA_MVIEWS.STALENESS? How to fix it?

Version 11.2.0.4.  I have a nested mview that after refresh using DBMS_MVIEWS.REFRESH_DEPENDENT on the master table shows 'UNKNOWN' staleness.

There is a table, LICENSE which has a mview on it:

SELECT LICENSE_ID, MAX(ROW_CHANGED_DATE), MAX(ROW_CREATED_DATE), COUNT(*) FROM LICENSE GROUP BY LICENSE_ID;

Then a nested mview on this mview and several others like the above:

CREATE MATERIALIZED VIEW "DATA_ENTRY"."LICENSE_TABLE_DATES" TABLESPACE "DATA" PCTFREE 10 INITRANS 1 MAXTRANS 255 STORAGE ( INITIAL 160K BUFFER_POOLDEFAULT) LOGGING PARALLEL (DEGREE DEFAULT) USING INDEX TABLESPACE "DATA" PCTFREE 10 INITRANS 2 MAXTRANS 255 STORAGE ( INITIAL 160K BUFFER_POOL DEFAULT)REFRESH