My thanks for Swagmeister for figuring this out!
There are a couple of UI mods that allow for Ultrawide resolution support. I don't have an Ultrawide monitor myself, so I can't be 100% sure if this fixes every UI element. To enable Ultrawide support, open the UI separator in MO2, and activate Ultra Wide Interface (21x9) and No Rainbox - 21-0 Rainbox Fix.
When in game, you will likely need to head into the Mod Config menu > FallUI and tweak some of the widgets to fit your screen space properly.