I attempted 3 root grafts on the branch pictured, only one took. It was awhile ago, but I remember being slightly surprised at the lack of new root development from the branch layer; there were 3-4 minor roots coming from the branch itself, at best. I'd guess as soon as the branch detects it has some functional roots, energy and development are primarily focused on wherever those are. If I had the time, I'd try starting a few air-layer attempts that I let go for several weeks/months before I attach any root grafts. If the branch had the beginnings of functional roots before the root grafts, maybe it would be different
I'll still attempt 2-4 root grafts on the next air-layer, but I'm going to select a smaller branch and/or a less-vigorous variety. I'll remove most leaves when I separate the air-layer next time and limit new branch/leaf starts, too. Grafting mangoes is definitely easier/more reliable, but I'm curious to see how an air-layered mango develops.