How to read cubemap from realtime reflection probe

Ok, I tried replicating what you are doing by rendering a probe via script, with individual faces, and set back this probe texture to an other one.

Take a look at this doc page : https://docs.unity3d.com/ScriptReference/Rendering.ReflectionProbeTimeSlicing$$anonymous$$ode.IndividualFaces.html

It mentions that updating the cubemap will take 14 frames. What you need to do in your case is to call _probe.RenderProbe() 14 times before trying to get _probe.texture. This will allow all faces of the probe + mips to get correctly rendered and available in _probe.texture.

Best thing to do is to use the render ID returned by _probe.RenderProbe() to check if it is finished rendering with _probe.IsRinishedRendering(int renderID), or set your probe to render in one frame.

Here's a the script I made for example:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.Rendering;
 
 [RequireComponent(typeof(ReflectionProbe))]
 public class ReflectionCopy : $$anonymous$$onoBehaviour
 {
     ReflectionProbe reflectionProbe;
     [SerializeField] ReflectionProbe target;
 
     int renderID;
 
     // Use this for initialization
     void Start ()
     {
         if (target == null)
         {
             enabled = false;
             return;
         }
 
         reflectionProbe = GetComponent<ReflectionProbe>();
         reflectionProbe.refresh$$anonymous$$ode = ReflectionProbeRefresh$$anonymous$$ode.ViaScripting;
         reflectionProbe.timeSlicing$$anonymous$$ode = ReflectionProbeTimeSlicing$$anonymous$$ode.IndividualFaces;
         Destroy( reflectionProbe.texture );
 
         target.mode = ReflectionProbe$$anonymous$$ode.Custom;
 
         renderID = reflectionProbe.RenderProbe();
     }
     
     // Update is called once per frame
     void Update ()
     {
         if (reflectionProbe.IsFinishedRendering(renderID))
         {
             target.customBakedTexture = reflectionProbe.texture;
             // Render Again
             renderID = reflectionProbe.RenderProbe();
         }
     }
 }