WorldToScreenPoint returning VERY high values

So I am trying to implement quad-tree terrain. I want the terrain chunk to subdivide into 4 child chunks on the condition that the chunk is taking 25% or more of the screen space.

Here is the code I use to determine that (found elsewhere on here):

 private void CheckForSubdivision() {
         Vector3[] corners = boundsCorners;
 
         float minX = Mathf.Infinity;
         float minY = Mathf.Infinity;
         float maxX = -Mathf.Infinity;
         float maxY = -Mathf.Infinity;
 
         for (var i = 0; i < corners.Length; i++) {
             var corner = transform.TransformPoint(corners[i]);
             corner = Camera.main.WorldToScreenPoint(corner);
             if (corner.x > maxX) maxX = corner.x;
             if (corner.x < minX) minX = corner.x;
             if (corner.y > maxY) maxY = corner.y;
             if (corner.y < minY) minY = corner.y;
             minX = Mathf.Clamp(minX, 0, Screen.width);
             maxX = Mathf.Clamp(maxX, 0, Screen.width);
             minY = Mathf.Clamp(minY, 0, Screen.height);
             maxY = Mathf.Clamp(maxY, 0, Screen.height);
         }
 
         float width = maxX - minX;
         float height = maxY - minY;
         float area = width * height;
         float percentage = area / (Screen.width * Screen.height);
 
         if (percentage >= Terrain.MAX_CHUNK_SCREEN_PERCENTAGE) {            
             Debug.Log("Chunk " + transform.name + " uses " + (percentage * 100) + " screen space; subdividing");
             Subdivide();   
         }
     }

boundsCorners is an array of 4 vertices that represent the 4 corners of the plane that make up the current chunk.

This works perfectly at first. As I move my camera closer to the terrain, the chunks are subdivded further and further. However...

After about the 5th subdivision, the screen space being used always registers as 100%, and unity hangs. Debugging showed me that when this happens, the corner variable has values like this after being processed by WorldToScreenPoint:

BEFORE:

• x: 2.605358f

• y: -1.97875226f

• z: -4.22133827f

AFTER:

• x: 126353.117f

• y: 54890.8555f

• z: 0.00229765475f

These are clearly WAY off the boundaries of my screen! What could possibly cause this? As far as I understand; the values should be within the range of my viewport resolution no?

It is worth noting that this terrain is spherical, and as such we will almost never be looking at it from the global x/y/z orientation (aka: camera will almost never point at -Vector3.up for instance).

I'm just trying to understand why values like this are being returned when I get very close to the mesh, and also interested in hearing about alternative approaches to get the same result.