In the 2020 iteration of the AMD Radeon Software they have removed all the settings to configure AMD EyeInfinity. Which means when you play games with a mix of different monitor setups, you’ll see gaps and black areas by default, normally on your largest screen. Something like this:
To fix this you need to find the old Catalyst Control Panel, which is now secretly hidden away.
When AMD EyeInfinity is enabled, head to C:/Program Files (x86)/AMD/CNext/CCCSlim and find the CCC.exe app.
This app will only launch when AMD EyeInfinity is enabled. When you launch it, you’ll be able to find the “Resize Desktop” option. Selecting “Expand” will fill all your screens properly.
Here is a video of how to do it!