Can you show us the result for two values that you mentioned in your formula for =ReportFilterSummary()
i.e. I need to see the results when your condition qualifies for Pos(ReportFilterSummary();"Rep Period Equal Current Month")>0 and the other one when it fails.
I want to see the string that is being returned by =ReportFilterSummary() for those two conditions.
Meanwhile you can just try below code.
=If(Pos(ReportFilterSummary();"Rep Period Equal Current Month")>0;[Rep Period];Substr(Substr(ReportFilterSummary();Pos(ReportFilterSummary();"Rep Period Equal");25);18;7))
Regards
Niraj