Summary
Transaction logs for Microsoft Structured Query Language (SQL) Server 2005 and later are only truncated after incremental backups and not after base backups.
Resolution
This is a design decision that helps to better protect data. You can find more information in Microsoft documentation.
Refer to:
msdn.microsoft.com/en-us/library/ms189085(SQL.90).aspx
and
The Catalogic Software Data Protection Software 4.0 for DPX Solution Reference Guide. To access this manual, log into the Catalogic Software support website (MySupport) https://mysupport.catalogicsoftware.com/ and select DPX > Reference Guide.