I have two questions:
- Can you set that resolution on the desktop?
- Does the game support borderless windowed?
If the answer to 1. is yes, then it should work in borderless windowed as well.
If it doesn't work ingame but only on desktop, then the game either isn't setting the window parameters correctly (if it's actually borderless windowed) or it's actually just exclusive fullscreen.
Lots of games nowadays don't even bother naming their modes properly anymore so "Fullscreen" now actually means borderless windowed.
Also I recommend DXVK. Just try it for giggles, it may magically fix the issues.
(Also that res isn't 21:9, that's even wider, because the closest to 21:9 would be 2560x1080, but that's a minor nitpick)