Thats the thing, its doing that because of that its not using the 21:9 ratio but the 16:9, but it results in that its UI is cut off at the bottom or top because of how it scale with the ratio.
Ive played a game that had similar problem, and the only way it could be resolved was that they needed to know where it scales when it try use 21:9 resolution types in order for them to readjust the UI to fit those resolutions.