It has, in fact, been solved. People just don't like the answer so reject it. I recently saw it happen to someone who lives a few blocks from me. They had a nice big clump of what appeared to be Pink Satin (it's hard to say for sure since the Sass brothers introduced several similar irises) growing in the strip between their sidewalk and the street. There's always a number of bee pods in the clump when it's done blooming that don't get removed. In 2018, a single stalk appeared in the middle of the clump with white flowers and a second one had yellow flowers. These were obvious seedlings growing within the existing clump. The homeowners apparently didn't notice because they didn't make any effort to remove the seedlings. Last year, there were a lot more white or yellow flowered stalks. By this spring the clump had about 75% white flowers and 25% yellow flowers. The seedlings were able to completely overrun the existing clump within two years of first bloom. For anyone not paying close attention, that would seem like an instantaneous transformation.
There's a couple of reasons why the supposed transformation always seems to be from purple to white or yellow. The first is that white or yellow flowered seedlings will pop up from all sorts of different colored parents. For example, if you cross a pink iris to a blue iris, you'll get a boatload of yellow seedlings, fair number of white seedlings, and a very small number of other colors. Put simply, it's easy to get white or yellow flowered seedlings from purple parents. It's more difficult to go the other way. White or yellow parents don't produce purple seedlings (unless the pollen parent provides the genes for purple of course). The second reason is that when a purple seedling overruns an existing purple clump, people don't notice. When a white or yellow seedling overruns a purple clump, it's hard to miss.
The reason why this seems to mostly happen to older irises is also pretty straightforward. In modern TBs, the gap between the fall petal and style arm is usually so large that it's difficult for bees, even big bumblebees, to deposit pollen on the stigmatic lip. Bee pods are much more common on older varieties.
