Hi Chuck,
I took a closer look at the fan with the bloom and the inner leaves do look considerably shorter than the outer ones, so maybe it had paused and I just hadn't noticed. It does have two small increase fans that have been growing pretty well, but then so does one of the other siblings and this sibling definitely has active leaves growing in the center of its main fan.
Maybe your "facilitative vernalization" concept would apply though since there definitely hasn't been any short day or cold temperature to trigger that stunted stalk.
Thanks for your thoughts on the matter. I'll keep you updated on how these guys behave as mature plants outdoors.
Take care, Tom

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Chuck wrote:
When an iris plant reaches maturity, it will stop producing new leaves, allthough the leaves that are there will continue to increase in size for awhile.

A secondary factor can be the increases. Usually the increases don't start to grow until the mother plant has shifted into producing a flower stalk.

Ther seems to be a type of (re)bloom, that I call "facilitative vernalization". This occurrs when a plant doesn't absolutly need vernalization to bloom. The plant will initiate bloom after an extended time at maturity at a temperatire that facilitates growth.

Chuck Chapman