Just out of curiosity, why would a seedling grow upside-down, with an
abortive root sticking up into the air? Did I do something wrong,
such as plant it too shallow, or is it just a "bad seed"?

I turned it right-side-up, as it was the only one that sprouted in
that pot. If it grows, should I make sure not to plant any seeds I
get from it, since they might inherit its poor sense of direction?

If it matters, the plant is Calliandra eriophylla, pink fairy duster.

--
Jerry Friedman