In fact, when backflushing I notice a steady stream pouring out of the hole.
This is NOT correct but, essentially, the only "harm" is that the drip tray is being overused.
The way to fix all of this is to temporarily adjust the pump pressure up to 12 bar while "brewing" with a blind filter in place. It doesn't need to be exactly 12 . . . anywhere from 11-12 will suffice. During this event, adjust the OPV such that it allows a drip . . . drip . . . drip into the drip tray. One or two drops per second seems reasonable.
Now readjust the pump relief/regulating valve to show 9.0 bar on the gage while operating with the blind filter. You're done . . . happy brewing.