itch.io

Just by curiosity, and I'm throwing an oddball to try and solve a possible cause of that sort of issue from experience, are you using a Nvidia GPU? And if you are, could you check your Nvidia control panel, in manage 3D control settings, then in the program settings tab there should be a drop down menu, inside of which you can find the game listed.
After selecting it, set it to 'use global settings' in the 2nd option (which makes it use the game's default settings). My thought process is that if it wasn't set to that, it might've used custom Nvidia processing settings and been forcing its own V-sync, which if inappropriate to the game played could create tearing and make all sort of funky stuff happen to some shaders. Sorry in advance if that doesn't solve the problem, it's just a thing I'm suspecting, it's very unlikely to do this on its own unless the user themselves have overridden default Nvidia settings manually, and while I doubt you have done that, it's better to be safe than sorry.

EDIT: Oh and, while you're doing that, it's a good opportunity to check if the game is in fact using your main GPU in that same window, if you're on a laptop often there's switcheable GPU options, and the game could've defaulted to your Intel GPU instead which is used for browsing and general utility, it has a lot less horsepower than the other one and perhaps could cause that sort of issue. Sorry for the long paragraphs of explanation, I'm just trying to find a swift solution to your problem since it seems rather frustrating, and since you said OBS wasn't picking up on the issue there's a good chance the issue is coming from your own device's display rather than the game itself.

Regardless that might be a serious thing to consider though, since as you said this was so bad it could cause seizures in some people. And this last paragraph is more directed toward Chubberdy; if it happens on one display it could happen to others, and if the dithering is in fact the cause, then it might be wise to either change the current dithering method used in game to something that would keep resolution fidelity, if it's a post-processing shader perhaps adding per-pixel antialiasing in the loop, FXAA would be my recommendation as it only uses the X and Y derivatives of an image and cuts them at the middle point which wouldn't affect the dithering effect.

If the dithering is a texture and it'd be a hassle remaking it however, I can only recommend adding some screen-wide anti-aliasing by default in the options sadly, I can provide a GLSL script that would do just that if you need it, sorry if I'm overstepping a bit by recommending this, I just want to help if I can, not trying to tell you how to make your game or anything, it's just that light sensitivity is a serious matter and is a thing to take seriously given the game has no warnings.