I have implemented rewarded ads in my game. The test ads load and play correctly, but they are not skippable as the 'X' button is always grayed out. As a result Google has rejected the app with one of the reasons being:
"Ads that interfere with normal app use that are not closeable after 5 seconds."
This seems to be a common issue as this active GitHub issue shows:
I have only tried this with the test ads, as the app is not live on the Google Play Store yet. But the Google Policy Team attached screenshots of what seem to be live ads, that are also not skippable. So this issue does not only effect test ads.
I am currently using the newest Ads SDK version 21.1.0 and tried different older versions down to 19.7.0, which was the earliest version that worked without changing my code.
This is one of the screenshots the Google Policy Team sent me, notice the grayed out 'X' Buttons.