Problem with OnUnityAdsShowFailure showing Unknown Error

Whenever I try to load and show a rewarded ads in Unity for the first time (every time the game being opened), it does not return OnUnityAdsShowComplete(). Instead, it shows OnUnityAdsShowFailure() with the UnityAdsShowError Unknown Type.

It occurs in a particular Android TV device for every first time I try to watch a rewarded ads (NOTE: The ads did show and there should be a reward after that, but it didn't lead me to OnUnityAdsShowComplete).

Does anybody have the same encounter and has any idea how this can be fixed?

Edit: I'm currently using Unity Ads Version 4.3.0 and Unity Version 2020.3.2f1