It looks admob will have a soft ban of 30 days that publisher doesn't know. I have a game that the match rate dropped from 80% to 50% from Jun 9 since I added the Facebook Audience Network as mediation. Then, I tried to remove the Facebook Audience Network from the mediation and the match rate raised back after a month. Unfortunately, the match rate dropped again from 9 Sept. It looks to me the my dropped is not related to the adding of Facebook Audience Network.
I've noticed that in the low match rate period, I didn't get any game related ad. The most ad that I seen is Uber.
Do you know how to gain back the match rate? Now, I add back the Facebook Audience Network to protect my income by set it a very low ecpm US$0.02 and pray for admob give me back the match rate next month.
The following is the summary.
May - added Facebook audience network
Jun 9 - match rate dropped from 90% to 50%
Jul - removed Facebook audience network from admob mediation
Aug 11 match rate went back to 90%
Sep 9 match rate dropped from 90% to 50%