Just wanted to throw my voice in the ring and say that I am having the same issue here. I thought it was just an ultrawide issue at first but after seeing this it seems it's a bug unrelated to screen ratio.
Also, the reason it sucks for me is that if the game is in windowed full screen, I can get full native resolution, however, the game is super choppy no matter what setting I put it on. When I switch to literally any other option the game runs much smoother in comparison but I get other weird behavior. If it helps I will try to detail the specific behaviors of each resolution setting for me.
With my native screen resolution set to it's native 3440X1440:
Windowed Fullscreen - Grants me access to native 3440X1440 21:9 resolution, however, performance is super choppy and headache inducing.
Fullscreen - Performance is much smoother, however, my max resolution is 2560X1440 and it stretches it to fit the screen. I do not have access to 21:9 resolutions.
Windowed - Performance is much smoother but my max resolution is 2293X1433 and I don't have access to 21:9 resolutions.
Now, even though this was bad I was still determined to play so I figured, I'll set my resolution to a 16:9 ratio and see if that helps...
With my native screen resolution set at 2560X1440:
Windowed Fullscreen - Grants me access to my custom resolution of 2560X1440. Performance is still choppy and headache inducing.
Fullscreen - Performance is much smoother, however, my max resolution is 2560X1440 and it stretches it to fit the screen as if it where 21:9 ignoring the pillarboxing of my custom resolution.
Windowed - Performance is much smoother, however, sets my max resolution to 2133X1333 and is displayed in a window. Resolution doesn't change if I maximize the window, just stretches the window and game to fit within the boundaries of the bounding box. It obviously looks horrible due to the loss of resolution and stretching.
Even weirder, I stream from time to time and my capture card is in the streaming PC which my wife uses for work at times. As I was fooling around with the resolution settings for the game my wife sat down and turned on the streaming PC and my screen went blank for a moment while the capture card from the streaming PC booted like it normally does. Well, as soon as my screen came back to the game all the options for the Display resolution settings in the game where different and at even weirder resolutions for my max options. ALL of them under my native res. So I went in to windows and set my screen to display only on one screen and went back to the game and voila, all the resolution settings where back to how they where when the capture card was off. All the tests above where done after I did this.