Interesting for me too!
I did E-mail
[email protected] in 2008, with several questions about the matter, but received no reply.
Have no idea of the prevalence of the reversion, which I've not seen mentioned on garden commercial or garden journalism sites.
However, have seen it mentioned by another gardener.
Had four of these plants, two started to show this reversion in the second year and the other two, by the fourth.
One plant is showing up to 40% reversion now (five years later).
May eventually start pruning out reverted branches/stems, though haven't wanted to so far.
Have one 'Candy Floss' (parent plant), which is also very hardy, but not as vigorous as 'Peppermint Twist'.
Last year was dividing the 'Peppermint Twist' plants and moving portions around.
Whatever the case, phlox are my very favourite summer perennials.