Hello Aryeh,
I am facing the exact same issue as mentioned by Pietro. Following are the steps that have consistently reproduced the issue for me.
1. Open the app
2. Press home button to push the app to background
3. After about 2 hours, re-open the app from the recents (the app is not killed during this time frame)
4. Blank/Transparent AppOpen ad is displayed now
Version - com.google.android.gms:play-services-ads:20.6.0