I believe you have already highlighted the issue - the value is reverting back to its "default" value. 🙂

If you check the parameters on the sub report (and parent report) by adding text boxes on the report with its value: =Parameters!YourParameter.Value

You can determine if it is being updated by the parent report or not - this should help you troubleshoot the actual issue when passing parameters between the two report.

HTH

GSC