I have noticed that some apps weren't working properly, such as Whatsapp and audio recording apps. Background task clear function was preventing them from working properly by killing the apps whenever the phone wasn't being used.

The solution to this is to use either use the whitelist for certain apps (Whatsapp + recording apps), or disable Background task clear altogether. How this interacts with Android's Doze/Battery optimization function, I'm not sure. At least I'm receiving my whatsapp notifications now, and the recording apps aren't being closed whenever the phone is inactive.

Just posting this because I don't see many mentions of this feature in this particular phone, though other chinese phones do have this feature (or a similar one).

Hi friend, can you please tell your location and send the video or picture for reference?

I noticed the same issue.
Mi locación is spain