The issue is NOT resolved.
Otherwise, why would I have written "Please fix this issue." ...
It's just that I've found a workaround, as a USER. Not as a developer. As a developer, I saw that the ads aren't showing anymore. As a user, what i did is to reset the Play-Services. No real user will do such a thing, especially to show ads.
And again, what do you mean "there might be an issue with the package name" ? What package name? I'm talking about 2 apps that work fine on the Play Store, with the exact same package name as before (I can't change package name anyway), on one device, yet on the Nexus 5x both of the apps failed to show ads.
This clearly shows that something is wrong in the SDK or on the Play Services.
The device isn't blocking ads. I didn't install any such app or modified the OS in such a way. Not only that, but resetting the Play Services proves it can't be the case.