The short explanation: your screen is too long.
The longer one: The battles fill the screens which have a 16:9 ratio. Your Galaxy S8 has a 18.5:9 ratio. For screens which are too “square” there are black borders on the left and right sides.
For the too long screens: maybe something has changed in the latest version, but it wasn’t better before. I don’t know if there were black borders instead of the brown ones in your screenshot or if the image was scaled on the vertical axis, or if there was some extra space between the elements (e.g. between the board and the battle items), but it couldn’t have been better before. (though if you have a screenshot of the battle screen before the update please post it, I’m curious)
I think you’ll soon get used to the new layout and it won’t seem disturbing.