I didn't mention it clearly: The sample works fine.
Only on the real app, where we have both debug variant and release variant, I started getting the issue (on debug variant)
Using the test ad unit ids on the real app, it started recently to always fail.
Using real ad unit ids works on the real app, and I want to avoid it for debug variant.
I prefer not to add test devices, if possible. I have a check that if it's debug variant, just use the test ad unit ids.
BTW, something is wrong with this website's email notification mechanism.
I couldn't see any link to here. I had to find it myself, manually.
Please also avoid writing my name and email here. I have enough spammers already.