You can read the research paper here:
and here:
Of course it is very technical language intended for peers. This is how I know what type of mutation it is.
As for the breeding of Senbazuru, it is one of a few satsuki cultivar where the parents are unnamed but the grandparents are named. The parentage is found here:
Satsuki Azalea Bonsai
satsukimania.com
Since the mutation is recessive, the parents of Senbazuru were very likely carriers of the mutant gene, but had normal flowers themselves. As is described in the research paper, where the F2 (second) generation had the mutant flower 25% of the time.
I have never heard of this type of mutation reverting. Narrow petal cultivar like Kinsai have a tendency to revert to normal flowers. So it seems that this mutation is very to extremely stable.
The research paper says that a retrotransposon has inserted itself in the AP3/DEF gene, interrupting and breaking the genetic code of the gene, causing a loss of function, resulting in the mutant flowers.. Theoretically, a retrotransposon can 'jump' out again, restore the function of the gene, and result in normal flowers once more. But apparently this retrotransposon doesn't jump. At least, I have never heard of it doing so or seen a picture of a Senbazuru or Chojuho with a branch that had normal flowers.
The papers don't comment on the stability or why this retrotransposon doesn't jump. They do call the mutation 'stable'. So I wouldn't worry at all. It behaves differently from Kinsai.
This info is mostly a curiosity and doesn't matter if you just grow it for bonsai or flowers. However, if you want to do breeding for this long-lasting flower mutation, this info means that if you cross two of these mutant flowers, all offspring will have the mutant long-lasting flowers. However, if you cross them with a normal azalea, all will have normal flowers. But all will also be carriers of the mutant gene. If you cross the F1 to obtain F2, there will be 25% odds of the long-lasting flower appearing. And when crossing back a F1 seedling to a mutant flower type, you will have 50% odds the long-lasting flower mutation occurring.
So for those who want to breed long-lasting flower cultivars, I would recommend crossing either Chojuho or Senbazuru with your favourite normal azalea. Grow these seedlings which would be the F1 (first) generation, all without the mutant long-lasting flower. Then cross these F1 seedlings back to Senbazuru. Senbazuru is preferrable to Chojuho, because Senbazuru is already a hybrid. Chojuho is likely a species form of Rhododendron indicum. So crossing with Senbazuru will likely result in more diverse offspring. There are no purple or multicolour or variegated long-lasting flower cultivars.