For some reason when the game runs in the regular viewport (not it's own) a grey haze slowly appears. This never used to happen. The level architecture (code) changed recently and then this started happening. However, none of the assets or visual build configurations were changed. This problem does not occur when the game is running in a separate viewport, but the fact is it wasn't an issue in the default viewport before. Does anyone know what is causing this and how to fix/prevent it?
From the information you provided, this could be an issue with a semi-transparent UI element scaling weirdly.
You can check if there's a faulty widget using the Widget Reflector: Window > Developer Tools > Widget Reflector (that's the last one in the list).
From there, activate the "Pick Painted Widgets" and hover your cursor over the "hazy" area, and it will tell you if there's any UI Widget there. (press Esc when you're hovering the element, so you can then look in the hierarchy tree of the Widget Reflector without changing the selection)
Cheers!
