We have been having an issue on the report UCR.
I created two Datasources with the same definitions and start a build. On finish the count of this similar datasources preview varies.
I'm not certain - I would expect two identical data sources to have an identical number of rows. The issue could be one of timing (ie, 401 cases came in between the first and second screenshot), but I'm imaging you've ruled that out already. More plausibly, this could be due to infrastructure issues, for example your UCR pillow could still be behind. There may have also been errors in the pillow process, such as timeouts when fetching or saving a document. I'd check monitoring and logs for errors.
There is also a management command find_datasource_mismatches, which might be able to shine more light on this, though I'm not sure it will be able to handle a UCR of this scale in a timely fashion.