Handles.DrawSolidRectangleWithOutline is being given a valid rect, and is drawing correctly.

The problem is coming from this line:

labelRect = HandleUtility.WorldPointToSizedRect(pos, labelContent, labelStyle);

HandleUtility.WorldPointToSizedRect is not doing any special handling for frustum culling, so when given a point behind the camera it still returns a valid rect.

Arguably this could return Rect(inf, inf, 0, 0,) or similar when given out of bounds positions, but that would be a breaking change for anyone already using this method, and really I'm not sure it leaves us in a better position. DrawSolidRectangleWithOutline is not typically used in GUI blocks, and all existing Handles functions that draw in GUI space (ex, Handles.Label) already perform a check prior to rendering that they are in front of the camera.

I would suggest doing the check that other Handles methods when drawing GUI from a world point, ex:

Handles.BeginGUI();
if (HandleUtility.WorldToGUIPointWithDepth(pos).z > 0f)
{
Rect labelRect = HandleUtility.WorldPointToSizedRect(pos, labelContent, labelStyle);
Handles.DrawSolidRectangleWithOutline(labelRect, labelRectFaceColor, LabelRectOutlineColor);
}
Handles.EndGUI();

Handles.Label(pos, labelContent, labelStyle);