Am I late to the party??? Please pardon my extreme tardiness, and I hope I am not being redundant as there may be other threads in this website which may also explain why JD's dough had yeast leavening 14 days later.
There are probably millions of different strains of yeast in the environment, all with different optimum temperatures for their specific "thriving" conditions. Some of them share this optimum temperature with each other, and some of them differ greatly. What you feed the yeasts matter too, and many other factors as well, but we'll stick to temperature.
For example, in beer I sometimes enjoy a lager style beer which is "cold fermented" at the low 40's not because beer can't be brewed at other temps, but because if you want a clear, crisp, somewhat mild flavored final product then one of the many varietals of the lager yeast will be your ideal helper, and if you want that strain to be the the dominant yeast (because there certainly are other yeasts present as well, just in smaller numbers), then the conditions "it" likes should be sustained.
Other times I enjoy an Ale style beer which has more character, darker appearance, stronger, more pronounced flavors. The yeasts that produce the by-products which determine these characteristics enjoy temperatures in the high 60's.
One time I was brewing a Belgian style Ale (with one of the many, many strains of yeasts), and the instructions in the recipe were incorrect for temperature. I was 10 degrees too high and it produced some terrible flavors, not from the Belgian yeast but from "another" yeast that enjoyed 80 degrees, and let me tell you it certainly "thrived"!.
JD's refrigerated dough was too cold for the ischia strain, but not too cold for another yeast (probably a lager type, if you wanted to make an assumption). 5 days was not enough for "that" yeast to reproduce enough of the wonderful by-products, but 14 days sounds about right. The ischia activity was probably near a complete stop during the whole time. By the way, an ale brews much faster than a lager because activity slows down at colder temps.
No mutation of the strains is happening, only the dominance of another strain is being observed.