I've found a serious privacy bug in system app of stock android 9.0 of Asus Zenfone Max pro m2. The problem is I am confused about where to file this bug? Should I go for Report bugs: Android or should I use ZenUI forums?
Since it is stock Android, it would be appropriate posting as directed in the link you included in the question. Make sure it's not already reported. If it is, you can either star (you would get mail updates) or add if your observations add more information
In addition, report on Asus official forums too, since you say it is a serious bug.