Hi aorsgl. The Power BI service doesn't support refreshing dynamic data sources because of how the service handles Power Query data sources. Unlike PBIDesktop, the PBI service expects to be able to inspect the M statically to determine what data sources are being accessed. It then can prompt you ahead of time for the credentials to use for these statically discovered data sources. However, because it relies on static analysis, it is limited in the kind of data sources it can discover. If it finds a data source function using dynamic expressions it doesn't understand, it blocks refresh of the dataset.
PBIDesktop, on the other hand (and PQ in Excel, as well as Power Query Online), prompts for credentials dynamically, as data sources are encountered during evaluation. This means it can handle fully dynamic data sources, since they work just like any other data source from its perspective.
Please feel free to vote for (or create) an idea about supporting refresh of dynamic data sources in the PBI service over on the PBI Ideas site.
Ehren
