Not sure if it's normal or not, so I just share the info here. It might be a bug, or just a lack of my knowledge on the topic.
After spending 2 days trying to build for the first time a mobile app, and in addition putting on it one interstitial or a reward add, I could never see the advertisement in test mode on my mobile.
In C3's preview (that for what I understand pretend to serve web based ads) all the mechanics works fine, while on mobile it doesn't. So the problem isn't my code.
Only when I gave up in trying to minify it, I finally got the ads. I tried once again with simple minification and it worked. While with the advanced it doesn't.
Originally I was also setting lossless and lossy format to WebP, before giving it up about the minification, but in the end, all my succesful tests were done with PNG and JPEG setting. I just now realised I should have tried also with no minification and simple minification, using WebP.
So that's it. Do you have the same problem?
If not I guess, it might depends on my mobile. So if I have this type of problem, also users might have the same issue too.