A very experienced little bird told me that this happens elsewhere,
shows up most often in darker colors (reds/blacks), but also some
others, end of season (hot, dry) and tends to be cultivar specific.
Good to know, so thought I'd share. Hate to see it in this particular
iris, will hope it's a root issue (i.e., inadequate root system) that
will be helped by more Z rebloom genes, which tend to <really> put on a
lot of root growth.