I'm not sure if it's supposed to be like that, but when I ran the game in 4:3 the screen was stretched: It might not be a bug or problem but come on if you're supposed to play in 4:3 or whatever aspect ratio that's not your monitor's aspect ratio then you'd not expect it to be a version of that aspect ratio stretched to the display's one.
I might suggest to add an option in the loader to chose if you want the aspect ratio to be stretched or not
But the actual bug relies on the mouse/cursor: when I'm moving my mouse to go somewhere in the menu using the stretched 4:3 aspect ratio, the game thinks the cursor position is in the original non-stretched 4:3 space (this of course applies to any other aspect ratio that stretches) when it should be at the stretched 4:3 coordinates. I don't think this is clearly said, I quite suck at explaining some things but, basically when you want to go and click a button you don't hover it in the same place as where it is located on the screen, basically the mouse coordinates and the screen coordinates don't align correctly when using those aspect-ratios.
I've seen this in other games if I can remember. I work solo on a game with a custom engine that SUPPORTS scaled resolutions and have encountered the same issue. Maybe I could send a video for more details in the problem I feel like I've explained this very horribly...