rollupMetrics and missing data


How does rollupMetric handle missing data when we use the ‘AVG’ rollupFunc?


I’d like to piggyback on this question for a rollupFunc of SUM as well.

If I have a list of Facilitys with missing plenty of missing data for an interval, but then call

    ids: ids,
    expressions: ['MyMetricWithMissingData'],
    start: '2018-07-01',
    end: '2019-07-01',
    interval: 'MONTH',
    rollupFunc: 'SUM'

the missing() value of the result is all 0. Is this a platform bug or is there a way in which we can get the missing values from the underlying metrics which were rollled up?