Unity Ads not working with AdMob Mediation

Hi! I have setup AdMob Mediation and I am successfully getting both interstitial and rewarded video ads from both UnityAds and AdMob in my app (both networks in testing mode, no revenue yet). However, it seems that the callbacks I register are not called when I get an ad from UnityAds, but they work fine when I get an ad from AdMob.
Here is the logcat output for an ad from AdMob:

07-02 14:09:38.701 27937 27937 I Unity :
07-02 14:09:44.612 4691 30635 I ActivityManager: START u0...

Unity Ads not working with AdMob Mediation