Hi,
According to Zhu's blog, "Record.FilterGroup(-1) cannot be used to add default filters in the OnOpenPage trigger."
One possible solution is to create a separate buffer and apply FilterGroup(-1) on it. Set your (Rec) to that buffer. Alternatively, you can use Or conditions to select the filtered record..

So you may consider using an alternate custom page and inserting the records on that page based on the filtered record in your buffer.. as Mr Zhu mentioned in his blog..
Regards