This filter is probably broken for some reason. It does not work for me either. I can not fix it, but I can offer you a workaround
If you select the "Found" status in Virtual GPS above the table in the "Group by", you will get the caches divided into two groups based on the Found status and do whatever more you would like to do (delete found, select unfound, show them in the map, whatever you like).
Additionally, under the "Manage list", there is "Remove found" option, if it is what you would like to do.