Are they all like that? Do other dahlia in the same area produce similarly stunted flowers?

If the answers are yes and no respectively, it looks like a somatic variation in that particular dahlia tuber. While I know a person or two who'd kill for that dahlia, you don't care for it--so remove it at season's end (or now) and don't use it again.

If it's only on one part of the plant, remove that stem as close to the base as you can. I once had a Sky Angel (lavender with white accents) produce white with faint lavender accents at the tips. I chopped that branch off and discarded it.

If all plants in the area do that, you may have a resource shortage in the soil, a disease there, or some sort of invader that warps the flowers.

Herbicide damage is my guess. Most commonly caused by contaminated compost or manure. Herbicide products affect dahlias in minute concentrations and numerous people have had problems. No cure but plants will have fewer symptoms as the season goes on and they should be OK next year.