It looks like chemical damage to me, as well.  But I will believe you if you say it's not from Roundup! 

I once had an iris friend give me a division of 'In Town' that had bloomed like this for her.  I grew it for 2 years, and the first year it was still like this.  Finally, the second year, it bloomed normal.  (hers was indeed due to chemical, though.)

We don't have Mountain Laurel here in Michigan.  Upon reading more about it, I understand there is some toxicity to the leaves and branches, right?  If you have this iris closer to the Laurel than the others, maybe it got some of the fallen leaves as mulch and absorbed a little of the toxin?  I'm only guessing here -- I'm not a scientist.  

What iris cultivar is it supposed to be ... do you know? It looks more like an orchid species now than most iris do, anyhow!

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