Why do my sprites turn into black squares after I run interstitial or rewarded ads?

**Godot version:** 3.5.2

**AdMob Plugin version:** for 3.2.2 or higher

**Issue description:**
My game sprites become black squares after I play interstitial or rewarded video ads.  I don't seem to have issues when I play banner ads.  

Does anyone know what's causing this?  The game works fine without ads. 

Thanks
