I'm not having this issue. But I am getting alerted all of the sudden on my Note9, since my recent Software update, that my Battery is draining faster than usual. And I never get a reason as to why! I have the normal 28 hour battery life I had before!
Make sure the charger is connected properly and see there are no any loose connection. If not Maybe the charger you bought was not genuine. For fast and better charging it is recommended to buy Samsung orginal charger from a authorized site or Samsung own site. If you are looking for a cheaper option buy from a brand that is recomend or is better like anker or other.