Может изменить строчку
Код
isGrounded = colliders.Length > 1;
на
Код
isGrounded = colliders.Length > 0;
Или сделать так.
Код
Collider2D[] colliders = Physics2D.OverlapCircleAll(new Vector3(transform.position.x, transform.position.y-0.5f, transform.position.z), 0.3F);
isGrounded = colliders.Length > 0;