Ah! Yes, thanks for the notice!
There is a "full screen toggle" in the pause and main menu. At the very least, I will make "windowed" the default option if it detects an ultra wide, and add a notice that ultra wide is not supported. Unfortunately getting ultra wide to work correctly would require a ton of changes to the in-game camera (to avoid messing up the background and showing unwanted elements) and many if not all UI screens. I will look into potential alternatives when I go over the settings for the game, which may happen over the next update.
I hope you can still enjoy the game in window mode, where resolution "snaps" to 16:9 as it is intended