In the Web Report Designer's objectDataSource wizard, once the "Preview data source results" (step 4) are ready and the Finish button is clicked, there is another call to the data source member but instead of using the Design-Time Values for the member, the values in the Value column appear to be used.

For example, If report parameters are already assigned to the data source parameters the string of the Value (e.g. "= Parameters.StartDate.Value") is used instead of the actual report parameter value.  I think the data member call to generate the data schema should be the Design-Time Values again. In order to get the data schema generated I had to use the same values for Value column as in the Design-Time Values value.

After the data source parameters are assigned to report parameters then it breaks the generation of the data schema later when the report is opened again in the Web Report Designer and there is an error (e.g. Error An error occurred while loading the data schema for "Widgets". ("Message"."String was not recognized as a valid DateTime.").