Thank you very much for your reply. So it is suspected that the Google Play Protect disabled the "Com.system.update" App? It makes good sense. We will check and reply.
Yes, from the appearance of that notification, it sure sounds like Play Protect identified it as potentially harmful. Apps that start with "com." are typically system-level apps, but I suppose it's possible that could also be malware trying to disguise itself as a system app. Or this could be a system app that Play Protect is incorrectly identifying as malicious (or who knows -- maybe the manufacturer installed malware on the device, knowingly or unknowingly).
EDIT: I did a little searching, and it does seem like that's a bogus "system" app that causes lots of ads:
If you go to Settings>Apps and tap Menu>Show System, can you find that app? If so, does it give you an option to Uninstall? A true system app can't be uninstalled.
Thank you very much for your help. Our engineers checked about the Play Protect settings exactly following your guide, and now we believe that the issue is solved. We will keep watching on the system for a while.
I tried to help our engineers by myself and found the two links as you listed. Due to my shallow knowledge of the Android system, I didn't come up with a solution. Thanks again for your help. I am very impressed with your fast replies and the efficiency for a rookie like me trying to get some help in Android Central!
Hope that I can come back someday to make a contribution to the forum!