Crests tend to want to buckle in the middle as they dry.
Any tissue exposed to the air will dry out faster than tissue not exposed to air.
Any tissue exposed to sun will also tend to dry out faster.
With that in mind, when your stock and scion are drying it helps to have holding mechanism that shrinks with the graft as the graft dries out. Even a little give can create an air bubble which allow more air in which makes drying faster.
For a graft this size, I would have personally used something like self-adhesive coban or pantyhose that has a little stretch to it.
I would also have thought about increasing the humidity a bit further for a few days during the initial healing via self-adhesive coban, a humidity dome, and or foregoing the full sun.
I know I talk about grafting in the full sun all the time... but then I have a particular way I graft when I do that.
But, if it works for you that is all that matters.
When I graft you graft we graft