It could be either a natural mutation (it doesn't look like fasciation in particular, but rather some kind of stunting) or the result of a viral or virus-like infection, but there is no way short of expensive lab testing to be certain. (Not all plant diagnostic labs stock testing kits for all plant viruses.)

If any herbicides were used near that plant, some of those active ingredients interfere with plant growth hormones and can cause similar symptoms to those of certain viral infections. Otherwise, there is no remedy, and you can either keep the plant and see if it regrows that way next year, or remove it out of an abundance of caution.

While some of the smaller types of mites like broad mites and cyclamen mites can cause plant stunting from their feeding, a microscope would be needed to confirm their presence (assuming they were still active and not dormant and hidden away in the heat).

For what it's worth, our plant pathologist is leaning towards this looking like Aster Yellows infection, and suggests removing that stem. To be fair, though, infections can be systemic and might be present in other stems not yet showing symptoms, although it's probably too early to decide to remove the entire plant(s) for one symptomatic stem. He is not able to verify the Aster Yellows suspicion, but that was his initial impression looking at the photos.

Miri