It's a problem with the game engine XNA which uses a graphic profile "HiDef" that only supports textures up to 4096x4096 resolution. That means if you zoom in and out in the game, the resolution of the game that is rendering is your screens resolution * (1 / zoom level). There's a ceiling cap in the game that protects the resolution to bypass 4096 and that creates really weir aspect ratios.
There could be some tricks to bypass this. Either try to override HiDef settings with some reflection hacks that's mention over Celeste github with the same problem:
https://github.com/NoelFB/Celeste/issues/5
I don't really recommend this "easy hack" since it would work on high res pc computer since it uses of DirectX drivers (I think it's only for DirectX, and not Direct3D). And it would be really heavy on your machine.
The other thing I could think of would be using two graphics device instances, splitted on two camera that moves across the screen with some matrix algorithm that keeps tracks of every assets transitions. This would be much cleaner way to fix this but takes a lot more time to implement. Just make sure to use the second graphical device if only the resolution bypass 4096, lazy implementation so to say so not every device need to handle a potential memory heavy solution.
4096*4096 is around 64MB of texture memory, so this solution should not be for everyone ;).
I've tried to fiddle around with my own mod to figure this out. But I ran into a dead end where I just gave up. But I wanted to share what I found out. Hope there will be any resolution of this matter.