SSAS : Update a dimension won't drop aggregation process index won't rebuild aggregation

I have a 'Employee' dimension which will be changed (modified) everyday, I made monthly partitions in cube and only process full the current month partition. Lately found that the past month's aggregation will not be dropped. Tired to 'ProcessUpdate' on this dimension and 'ProcessIndex' on partition but remained same. Also tried the setting 'ProcessAffectObjects'and 'ProcessIndex' again, still the same, tried both on lazyprocessing true and false with no luck. So my question is how to drop the stale aggregation on past month and rebuild them explicit ? It is a distinct count measurement, just read that 'DISTINCT_COUNT measure is not aggregatable', Is that the problem ?

Tried drop the index by using 'ProcessClearIndexes' in XMAL command, it worked fine and use 'ProcessIndexes' did rebuild the indexes and aggregation, saw them from the SSMS query execution message .

So might it only be related to the distinct count , just because it is a non-aggregation measurement ?