Media.net needs traffic from 3 countries: US / UK / CA, so if you have traffic from other countries, it is not well seen by them and may result in receiving information about "low quality traffic".
From what I was able to find out if you get a sudden drop in RPM to 0, it means you do not have or you have too little traffic from the previously given 3 countries. Unfortunately, in the case of media.net, even if you have high-quality traffic from other countries, this is practically irrelevant as 90% of media.net earnings comes from the US, therefore they focus on advertisers from this country.
Solution? Block any traffic from non-Tier 1 countries and you'll see better results. Additionally, you can talk to your account manager about activating "display ads" which will significantly increase your revenues.