Some of my bloodwork revealed subclinical lead poisoning. Part of the investigation was to take the dirty A/C filter in for lab analysis of the dust. I had lead in that filter, so I moved out. No idea where it came from.

EDIT: It was an apartment. FWIW, even new apartments can have other issues, like a brand new place my friend was working on had rampant mold issues (built & exposed throughout monsoon season). So you really have to be careful.

Who bought the place after disclosing airbourne lead dust in the house?

If it was a rental, GP would never know who moved in next. (I would describe leaving a rental as "moved out" and leaving a house I owned as "sold it". Given they used "moved out", I'd wager even-money it was a rental.)

In california if you sign a lease for a building built at least 20 years ago you probably get a boilerplate lead warning document as well. Its like a prop 65 situation where so many things are labelled that the label itself loses meaning.

Soils in many major US cities can have high amounts of lead. If you're not controlling how much dust is tracked in that very well could be detected.

Perhaps OP had a rental with an in-unit AC unit which is pretty common here in the US.

It was an apartment with central cooling.

if you are renting (in most / all? of the US), no way the next person would know.

Perhaps significant enough vaper from plumbing solder made it into AC unit or some other building or trade technique which involves some lead

Plumbing solder is overwhelmingly lead-free and has been for years. Even if leaded solder was used, the vapors from soldering are flux residues, not lead.