Libgdx FillViewport vs FillCamera unproject issue

Question

I have no idea why this happens. But I have the following scenario:

• Rendering a tiled map works fine.
• Rendering the sprites texture region works fine.
• Rendering the shapes of the sprites using a ShapeRenderer works fine
• Unprojecting from the fill viewport works fine.

What doesn't work:

• The box 2d lights when the camera is at the top or at the bottom of the map. (at the bottom they appear a little lower than what they should, and at the bottom, a little higher)
• Unprojecting from the fill camera.

Both issues do not occur when the screen resolution is 1.5. Which is the screen resolution that I use to create the fill viewport.

This is the code that I use to create the camera and the viewport.

fillCamera = new OrthographicCamera();
fillViewport = new FillViewport(Constants.CAMERA_PIXEL_WIDTH, Constants.CAMERA_PIXEL_HEIGHT, fillCamera);
fillCamera.position.set(Constants.CAMERA_HALF_PIXEL_WIDTH, Constants.CAMERA_HALF_PIXEL_HEIGHT, 0);
fillCamera.update();

public static final float CAMERA_PIXEL_WIDTH = 375F;
public static final float CAMERA_PIXEL_HEIGHT = 250F;
public static final float CAMERA_HALF_PIXEL_WIDTH = CAMERA_PIXEL_WIDTH / 2;
public static final float CAMERA_HALF_PIXEL_HEIGHT = CAMERA_PIXEL_HEIGHT / 2;

What could the issue be?