How to use AdMob Mediation with Facebook Audience in Xamarin Forms

I have a Xamarin Forms app (2.3.4+) with AdMob ads. I can see banners and interstitials in both Android and iOS apps. I'd like to enable AdMod mediation to show also Facebook ads, but mediation is not working.

Everything is configured server side. I've configured Facebook and Google, following the respective guides. I can see Facebook banners and interstitials if in the app I show them directly via the Facebook APIs and not via Google AdMob.

I guess the problem is app side. When I call InterstitialAd.Show() only AdMob banners are shown. For testing purpose I've set eCPM = 200$ to force Google to serve FB ads:

On the AdMob dashboard I see the N requests yet 0 impressions for Facebook:

In the logs I see:

07-06 12:54:13.390 I/Ads     (29542): Starting ad request.
07-06 12:54:13.395 I/Ads     (29542): Use AdRequest.Builder.addTestDevice("XXX") to get test ads on this device.
07-06 12:54:15.817 I/Ads     (29542): Trying mediation network: https://googleads.g.doubleclick.net/aclk?sa=...
07-06 12:54:15.831 I/Ads     (29542): Instantiating mediation adapter: com.google.ads.mediation.facebook.FacebookAdapter
07-06 12:54:15.836 I/Ads     (29542): Trying mediation network: 
07-06 12:54:15.844 I/Ads     (29542): Instantiating mediation adapter: com.google.ads.mediation.admob.AdMobAdapter
07-06 12:54:15.848 W/Ads     (29542): Server parameters: {"pubid":"ca-app-pub-XXX\/cak=no_cache&cadc=wb&caqid=YYY","gwhirl_share_location":"1"}
07-06 12:54:18.509 I/Ads     (29542): Ad finished loading.
07-06 12:54:18.722 I/Ads     (29542): Ad opening.

I'm using these packages:

// Android
<package id="Xamarin.GooglePlayServices.Ads" version="42.1021.1" targetFramework="monoandroid71" />
<package id="Xamarin.GooglePlayServices.Ads.Lite" version="42.1021.1" targetFramework="monoandroid71" />
<package id="Xamarin.Facebook.Android" version="4.22.0" targetFramework="monoandroid71" />
<package id="Xamarin.Facebook.AudienceNetwork.Android" version="4.22.0" targetFramework="monoandroid71" />

// iOS
<package id="Xamarin.Google.iOS.MobileAds" version="7.16.0" targetFramework="xamarinios10" />
<package id="Xamarin.Facebook.AudienceNetwork.iOS" version="4.24.0" targetFramework="xamarinios10" />
<package id="Xamarin.Facebook.iOS" version="4.24.0" targetFramework="xamarinios10" />

Any idea on why Facebook are never served by AdMob?

Thanks