Are you sure that the cambium is dead?
It looks like healthy new bark forming to me.
If you take a knife and just cut into that pink area carefully I think you may well find it is a healthy cambium layer.
The way that old bark has gone suggest that it has been removed by something like a mouse or small rodent. Or even damaged by being hit with something.
You say that another tree has suffered. What was the rootstock?
Stock 9 can shed bark shards similar to this and it seems normal. The cambium layer is not affected. Dwarfing stocks like 9 are very poorly rooted so any interference in the trunk area will affect the trees capability to draw nutrients up, but they will recover.
if you have had very hot weather lately then Sun Scald can cause damage similar to this too. Damaged thin bark peels off, but again trees usually recover well.
Bark loss can restrict growth levels but trees recover well normally.
There are two diseases that can cause damaged bark and cambium on the graft Union area,
First is canker..... Nectria galligena,
The other is Collar Rot.... Phytophora cactorum.
Both of these actually take the whole cambium and bark off from the hard centre of the trunk.
The damage normally is much more ' ugly' looking and both at that level on the trunk would eventually be fatal.
Collar Rot and Canker would kill the tree and heavy trimming and sprays of copper would only delay the inevitable.
However....I'm still convinced that your tree has had outside interference of some sort.
By the way.....Collar Rot gives itself away by smelling very strongly of vinegar.
I hope I'm right!!