Rigidbody.velocity Movement code produce wildly different gravity interactions

Main character movement using Rigidbody.velocity and two methods of control for player preference. The character moves via WASD (either locally or globally) and rotates to face the direction of the mouse pointer. I can confirm that the aiming script does not manipulate the problem, it is just this script.

The problem I am facing is that the two methods interact with the player differently, as intended, however while 'Scheme2' correctly follows physics (namely gravity, force modifiers etc), 'Scheme1' has this effect where the character floats extremely slowly towards the ground after moving off a ledge.

No amount of applied force or the absense of applied forces seem to affect the rate of fall. As stated, 'Scheme2' interacts as intended with gravity.

Why does 'Scheme1' fall at an absurdly slow rate?

My code for the movement script is posted below:

 public class PlayerMovement : MonoBehaviour 
 {
     public float speed = 0.0f;
     private Rigidbody PhysicsBody;
 
     //Relevant to current control scheme testing
     private bool Scheme1; //Default Control Scheme: Screen-Based
     private bool Scheme2; //Alternate Control Scheme: Pointer-Based
 
     void Start ()
     {
         PhysicsBody = GetComponent<Rigidbody>();
 
         //At the start of the game set Control Scheme to Default
         Scheme1 = true;
         Scheme2 = false;
     }
 
     void FixedUpdate ()
     {
         if (Scheme1 == true)
         {
             var PlayerInput = new Vector3 (Input.GetAxis ("Horizontal"), 0.0f, Input.GetAxis ("Vertical"));
             PhysicsBody.velocity = (PlayerInput * speed * Time.deltaTime);
         }
 
         if (Scheme2 == true)
         {
             //forward
             if (Input.GetAxis ("Vertical") > 0) 
             {
                 PhysicsBody.velocity = (transform.forward * speed * Time.deltaTime);
             }
 
             //back
             if (Input.GetAxis ("Vertical") < 0) 
             {
                 PhysicsBody.velocity = (-transform.forward * speed * Time.deltaTime);
             }
 
             //right
             if (Input.GetAxis ("Horizontal") > 0) 
             {
                 PhysicsBody.velocity = (transform.right * speed * Time.deltaTime);
             }
 
             //left
             if (Input.GetAxis ("Horizontal") < 0) 
             {
                 PhysicsBody.velocity = (-transform.right * speed * Time.deltaTime);
             }
         }
     }
 
     void OnGUI ()
     {
         if(GUI.Button(new Rect(50,50, 200, 50), "Default"))
         {
             Scheme2 = false;
             Scheme1 = true;
         }
         if(GUI.Button(new Rect(50,120, 200, 50), "Alternate"))
         {
             Scheme1 = false;
             Scheme2 = true;
         }
     }
 }