Have you tried to create your screen setup with the eyefinityPro app? You will find it in the installation folder of the AMD Driver (by default C:\Program Files\AMD\CNext\CNext).
I had the same problem the resolution is not right for some reason in the last few drivers. The EyefinityPro app is way more stable and has few more interesting features like adjusting black bars that can happen with some screens or resolutions with pixels offsets and it let you combine the displays in three different way Fill, Fit or Expand. If the screens are the same try the Fit mode.
Just as a comment, 7680x1440 on ACC with a 6650XT you are really optimistic, i can only run it around 75fps with a 6900xt and only 5760x1080p