I have noticed this happening multiple times now in different games. Even in cases where everyone on the team reports the griefer, there is no report feedback coming back. I feel like the system discards griefing reports for players who build Midas while maintaining a low death(refusing to fight or help team).
AA is the griefer who at 34 minutes had 1 assist:
I think it updates in batches. I got a whole bunch of action taken on griefers reports over the last week or so.