That would be a standard issue w/Data Blending. Tableau automatically treats Data Blending as a LEFT JOIN. I find it useful for bringing in disparate data fields that have some common data points for DISPLAY purposes but not for any extensive calculations. That's where it will bog down.
Is there a reason you couldn't do a JOIN within the Data Pane Window using the common/unique fields? Select your primary data source first, then go to Data Source then Add new data source (whatever you're secondary data source is that you're currently blending to). Tableau behaves much better w/JOINED or UNIONED data.
Also, without a workbook it's hard to tell if the calculated field you're working on will work in some other manner. If it's proprietary, can you mock some data up in a packaged workbook and post?