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This is an artifact of cross-database joins as a feature (aka "federated connections"). It is currently any of the data_connection.id values for that data source. See TS Data Connections for a pre-built data source to analyze this info. The logic is contained in the custom SQL. This will be changed in future versions to match the actual datasources.id value, as I understand it. Not sure when that will change, though.
It appears that this issue still exists in 2019.2 - will it continue to work (or not work) this way