[2021] Graphics Query

Check your graphics options in game for the Dynamic Resolution setting. If it's enabled, it's likely the cause. It dynamically downscales the resolution of textures to help the game maintain a stable FPS, which means when it happens it's due to an FPS drop from one source or another.

So if you disable it and it resolves the issue, you're liable to encounter framerate drops in any situation where it previously went blurry.