[Rigidbody] Vector3.ProjectOnPlane() issues on steep slopes.

Hi all,

I'm currently having a bit of an issue with slope movement and Vector3.ProjectOnPlane.

When moving up a light slope, my code behaves as I would expect, however when moving up a larger slope the movement direction seems to "drift" from the desired movement direction after passing through Vector3.ProjectOnSlope.

Example:
Desired Direction - Magenta
ProjectOnPlane Direction - Red
https://i.imgur.com/0xbAfSI.png (Was unable to upload as image, so here's an imgur link)

 Vector3 GetVectorOnSlope(Vector3 vector) {
         RaycastHit hitInfo;
         if (Physics.Raycast(orientationTransform.transform.position, Vector3.down, out hitInfo, 1f, groundLayer)) {
             Debug.DrawRay(orientationTransform.position, Vector3.ProjectOnPlane(vector.normalized, hitInfo.normal) * 2f, Color.red);
             return Vector3.ProjectOnPlane(vector.normalized, hitInfo.normal);
         } else {
             return vector;
         }
     }
  
 Vector3 movementDirection = (orientationTransform.forward.normalized * inputs.y + orientationTransform.right.normalized * inputs.x);
  
         if (stateHandler.IsGrounded) {
             if(stateHandler.IsOnSlope) {
                 Debug.DrawRay(orientationTransform.position, movementDirection.normalized * 3f, Color.magenta);
                 return GetVectorOnSlope(movementDirection) * movementForce * Time.fixedDeltaTime;
             } else {
                 return movementDirection * movementForce * Time.fixedDeltaTime;
             }
         }

I understand this is Vector3.ProjectOnPlane working as it should, I've tried rotating the vector back to the desired direction, but to no avail.

Any help with this would be appreciated, I'm sure there's a way to do what I want to do, but I'm stumped.

Cheers,
Slug