Using Admob mediation, why is my impression rate so low with Facebook Audience Network (bidding)?

Fairly recently I added Facebook Audience Network into my iOS app's AdMob mediation setup. I've confirmed that the Admob adapter is initializing the FAN adapter properly on device & using the Mediation tester that it shows all green checkmarks -- in other words, I don't see any obvious misconfiguration. The Facebook Monetization Manager does not show any configuration issues apart from low impressions.

However, when I look at reports on Admob for network performance, I see the following:.

1. Any ideas as to what I'm getting 0 impressions for that second row?
2. My understanding was that FAN has been bidding only on iOS for some time now, so what even is that first row, the one without "(bidding)"??

This turned out to be a misconfiguration between the ad type configured in AdMob (native) & FAN (native banner). Once I flipped the native banner to native in FAN, ad impressions improved as expected.