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Do you have data source filters that you don't know about?
Also, what kind of data source is it? (Excel? TDE? etc.)
If you were to create a brand new workbook that uses the expanded data source, what happens? (Don't copy sheets. Don't copy anything. Just start new. Attach to the data source new. Look at it there.)
If you can't share workbook (we would need just the TWB - XML definition, not the data) then log file, especially queries from it, can also provide some insights...
Without any of that - it will remain mystery!
It's probably not datasource filters since data source tab ("import tab") takes those into account before displaying data so something on the viz.