SQL Server Reporting Services (SSRS) will attempt to determine whether the data in the ChunkData table is still needed all by itself. If you review the error log, you should see messages where the SSRS engine is attempting to purge the data by itself using the following query:

DELETE ChunkData FROM ChunkData INNER JOIN #tempSnapshot ON
ChunkData.SnapshotDataID = #tempSnapshot.SnapshotDataID

I would venture a guess that the following error is being thrown:

ERROR: Sql Error in
CleanOrphanedSnapshots: System.Data.SqlClient.SqlException: Timeout
expired. The timeout period elapsed prior to completion of the
operation or the server is not responding.

If that is the case, you should monitor that instance to see whether there are blocking locks or other performance bottlenecks that are stopping the purge and tune accordingly.

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