The issue occurs when a report contains a subreport with InstanceReportSource set as its ReportSource. If the report definition in this InstanceReportSource contains data sources, they will be serialized as DataSourceRef objects instead of using their actual type names like EntityDataSource, CsvDataSource, etc.
The issue is reproducible only in Standalone Report Designer, i.e. serializing/deserializing the report definition programmatically would work as expected.
The issue is introduced in Telerik Reporting R1 2019.