I don't know if the OP or anyone else on this forum has figured out the problem but there's nothing at all wrong with your 16" MBP. There are only 2 major things that are causing the battery drain and possibly a 3rd one. And yes I have a 16" MBP with the AMD 5500 8GB card.
#1 issue is the AMD dGPU. It drains battery. Period.
#2 issue are the apps being used that activate the dGPU.
Here is my short list of apps that are doing this.
Chrome. Not only is it a resource hog but it activates the dGPU.
Spotify. Activates the dGPU. Seems like a simple app but it's really not.
Final Cut Pro X activates the dGPU.
Apple's Photos app activates the dGPU.
Adobe's Creative Suite. Resource hog and activates the dGPU.
Pixelmator photo editor. Activates the dGPU.
Connecting the MacBook to any external monitor activates the dGPU.
iMovie activates the dGPU.
Bootcamp. Running Bootcamp'd Windows works only on the dGPU and there's no way to switch it to the Integrated Graphics.
Playing most games activates the dGPU.
These apps cannot run smoothly using the Intel Integrated Graphics because MacBooks with dedicated graphics like the AMD 5000 series ship with lower grade Intel Integrated Graphics. 13" MacBooks come with better Intel Integrated Graphics to run these apps since they don't ship with a more powerful dGPU.
Lastly the 3rd reason for battery drain (but not as major as the dGPU) is keeping the screen brightness above 70%. If you're using the auto brightness control then this may not affect the battery unless you're using it outside or in uncontrolled lighting.
Without fail I get 11+ hours of battery on my 16" MBP when I'm using apps that don't activated dGPU such as Safari and Apple Music.
It's best for people to verify if the dGPU is activated. Apple Menu-About this Mac. If the Intel UHD Graphics 630 and the AMD Radeon 5000 Series GPU are listed then the app you are using will drain your battery. If you're unsure which ones then close all your apps and launch each one and verify if the dGPU is activated.
Lastly if you leave these apps open and just shrink them into the dock or close their windows they are still keeping the dGPU activated, so you will still experience battery drain even when you're using basic apps like Apple's office suite.
I repeat, there is nothing wrong with your 16" MBP's battery or the computer or the system management controller. These battery complaints have been going on for several years with MacBooks that have dGPU's. More apps today are using the dGPU so it's not always easy to achieve the advertised battery life, but nobody is being lied to here as someone suggested earlier.
If you absolutely need to use these apps at a constant it's best to plug in the Mac for long sessions.