Sphere behaves strange by jumping on diagonal Plattforms or at terrain

Hey, im using this code:

     public int lifepoints;
     public float speed;
     public float jumpheight;
     private float h;
     private Rigidbody rb;
     public bool grounded;
     
     void Start () {
         rb = GetComponent<Rigidbody>();
     }
 
 //Grounded
     void OnCollisionEnter(Collision other)
     {
         if(other.gameObject.tag == "Plattform")
         {
             grounded = true;
         }
     }
     void OnCollisionStay(Collision other)
     {
         if (other.gameObject.tag == "Plattform" && grounded == false)
         {
             grounded = true;
         }
     }
 
 void OnCollisionExit(Collision other)
 {
     grounded = false;
 }
 
 void FixedUpdate ()
     {
 
         /* ************Movements*************/
 
         // Move
         h = Input.GetAxis("Horizontal");
         Vector3 move = new Vector3(h, 0, 0);
         rb.AddForce(move * speed);
 
         // Jump
         
         if (Input.GetButtonDown("Jump") && grounded)
         {
             rb.AddForce(0,jumpheight,0);
             
         }
     }
     
 }

Grounded works pretty well, thats not the problem. The problem is, if my sphere is on a horizontal plattform without any rotation it works like it should. But if the sphere is on a diagonal plattform it sometimes jumps as twice as high as it usally does and should. This happens aswell if the sphere felt down on terrain under the diagonal Plattform.

I already tried different forcemodes and tried to multiply with Time.DeltaTime. Both did not really work.

Any ideas?

Maybe its not the code i dont know.

And im not sure but seems this first happend after patch 5.5

Help would be Great! Thanks!