You are using only World Location of ConvertMouseLocationToWorldSpace which is not enough. Take into account World Direction as well, as when you camera is at 90 degrees pitch, World Direction is simply (0,0,-1) vector. To take into account tilt of your camera, your actual intersection point would be World Location + World Direction * {some distance from camera into the world}. For the matter of fact not only tilt but position of the camera affects this as well.
I have a similar functionality in my project and what I use is GetLineVsPlaneIntersection (don’t remember exact name of the node), which I feed [World Location] as a start point and [World Location + World Direction * 500000] as end point. Another parameter for the plane is it’s height above some axis, you can set it somewhere above the ground. This will give you a point in 3d space where your mouse cursor intersects a plane and it won’t be obstructed by other geometry.
I can show you a blueprint when I get back home, if I don’t forget about it 