Hey, love your game, it's a nice mix of doom and half-life, which I suppose is quake. Either way, I genuinely thought it was an old game when I first saw it, so props. I had pretty much no problems the whole playthrough, and the bug I just got didn't hinder me at all. I had just gone through all the levels and gotten the secrets for all of them (on medium difficulty), and then I played through the cut levels, and beat those (with a little cheating). then I closed the game and realized I hadn't gotten the achievement for unlocking all secrets (I assume there are different secrets based on difficulties). But I wanted to make sure, so I started it right back up again. the game opened normally, but when I clicked level select, and chose my difficulty, I was brought to a screen that displayed the individual cut levels in a row, the pictures for them were just white. So, I clicked one of them, and when I spawned in, the entire surrounding level was white. certain objects could be made out, but otherwise I couldn't see anything. I could move and shoot just fine though. I figured it was just a mishap, so I left the level and was met with the same screen and the same white. So, I moved around a little bit, and then realized that there were jack-O-lanterns everywhere. That made me think this was an intentional bug and I had just inadvertently set it off. but I'm not sure how to make it go back either. and it seems like a really strange thing, just Jack o lanterns instead of items and blinding white. I guess that's all. I completed the whole game on normal, I got to play it, honestly, I'm just confused and would like to know if it is in fact intentional and if not figured you might want to know even though the game had been out for a while. Apologies if I've wasted your time. 
thank you for finding this 👍
I'm not sure how this happened: you have custom map completed screen
as the background of the level select screen which shouldn't happen 🤔
(the jack-o lantern just means the model was not found - these pumpkins are the default model
also the white texture means the texture was not found)
anyway if I understand correctly:
you played through the main game and the cut maps and this broke the level select? 🤔
also to answer your question: you get the same secrets on all difficulties
you got an achievement for beating the game on every difficulty but this
didn't went well because higher didn't unlock the lower ones
I kind of misunderstood why people want achievements 😓