The device that is having these weird behaviour is Huawei Matepad T8, HMS Core Version 6.3.0.326. On this device all the other ads are working fine.
I have made a build from Unity using hms plugin V 2.1.0 on unity 2020.3.26. The logs are from the unity apk.
[HMS] HMSAdsKitManager Loading Interstitial Ad.
[HMS] HMSAdsKitManager LoadRewardedAd
[HMS] HMSAdsKitManager Rewarded ad loaded!
[HMS] HMSAdsKitManager OnInterstitialAdLoaded
[HMS] HMSAdsKitManager OnInterstitialAdLoaded
[HMS] HMSAdsKitManager Rewarded ad loaded!
[HMS] AdsDemoManager ShowInterstitialAd
[HMS] HMSAdsKitManager ShowInterstitialAd called
[HMS] HMSAdsKitManager Interstitial Ad Still Not Loaded Yet!
the logs are from hms-ads-demo-java apk https://github.com/HMS-Core/hms-ads-demo-java.
I/HiAdSDK.InterstitialAdManager(27222): loadAd
I/HiAdSDK.InterstitialAdManager(27222): ad is invalid, content id:46348005
I/HiAdSDK.InterstitialAdManager(27222): onAdsLoaded, size:1
I/HiAdSDK.InterstitialAdManager(27222): loadAd
I/HiAdSDK.InterstitialAdManager(27222): ad is invalid, content id:46673268
I/HiAdSDK.InterstitialAdManager(27222): onAdsLoaded, size:1
I have test these 2 apk's on a different device Huawei P40 lite, and everithing was working fine. I have to mention that these two devices (huawei mate T8 and P40 lite) are real devices. The Country/Region Is not US.