I get that Rey supposedly force healed the kyber crystal. But as it stands right now there’s nothing in the actual movie that references that the kyber crystal is broken or that it can be healed. So to the average viewer it would just be confusing to see the saber go back to normal.

The last time we saw it it was split in half, so we can have it jitter with no further explanation. But if it changes partway through the movie then it would need an explanation, and we would be falling into the same problem the movie already has of explaining things with external sources instead of in the actual movie.

Edit: The moment I would have it be whole again would be when Rey crosses the lightsabers, since it could be inferred that it’s taking energy from Leia’s saber, and it would add punch to that climactic moment.