AddForce in Update

If you want to ensure that you're reliably applying your physics forces during FixedUpdate() while simultaneously basing it on an animation that plays during Update(), you could use a Queue<> (or List<>, on the same general basis).

In short, you add forces to be applied during the animation, then "play them back" during the next FixedUpdate().

 Queue<Vector3> qForces = new Queue<Vector3>();
 
 // Simplified example call
 void OnStateUpdate(/*etc.*/)
 {
     // etc.
     qForces.Enqueue([FORCE_TO_APPLY]);
     // etc.
 }

 void FixedUpdate()
 {
     if(qForces.Count > 0) // Only worry about this when the queue isn't empty
     {
         Vector3 combinedForce = Vector3.zero;
         while(qForces.Count > 0)
         {
             combinedForce += qForces.Dequeue();
         }
         // Make only a single Rigidbody.AddForce() call using the
         // sum of all forces gathered between physics passes
         rb.AddForce(combinedForce, ForceMode.[Impulse/VelocityChange/etc.]);
     }
 }

^{Edit: Efficiency improvement}