What would cause an iris flower to bloom but the stem never grew. It bloomed in the crotch of the leaves. And a stem didn’t develop.
Tall bearded iris was planted 11/17. When it began to bloom in July 2018, the flower never grew a stem.
Certified GKH Gardening Expert
Most likely it was a problem with the rhizome. There may have been a stunted stem, so short you couldn't see it.
I think I would dispose of the rhizome, or you could leave it and see if it grows normally next spring.