The property "pyImportedUser" is an unexposed column in the table "pr_operators". Once you expose this column, it should work. I tried it myself, and I could save the RD with this exposed column being added to the list of source.
Please open the "pyImportedUser" property ruleform. From the "Actions" menu items, choose "Optimize for reporting". This would expose the column in the table and also populate data from blob to this newly exposed column.