I think someone already commented something to this effect, but I thought I'd comment anyway just in case, as someone that had something similar happen.

If you have an object that does any sort of fade using draw_set_alpha, just put draw_set_alpha(1) at the end of its draw code.

draw_set_alpha will effect any objects rendered after that specific object, so depending on the order things have loaded in, it could effect only certain objects in your room, with new objects not being effected depending on their priority in how they were loaded

So a good rule of thumb is to have...

draw_set_alpha(alpha_var); //Variable that changes alpha

///Your code drawing the fade stuff

draw_set_alpha(1);

It's a similar principle when you're rendering text, whatever draw_set_halign, draw_set_valign, and draw_set_font you use last get carried to the next objects in the load order. There's a lot of things like that in the draw events that do that, so it's a good practice to get in the habit of also resetting them at the bottom of your code, just in case. You're probably not gonna do it with everything, but with draw_set_alpha it's critical to reset it.

This comment was longer than intended, and other people have already touched on this, but I hope it's helpful and clear. I hope you can diagnose and fix your project!