Turns out it's an old law, if you expatriated between 2004 and 2008, and spent 30+ days in the US within 10 years of expatriation.
> Further, expatriated individuals will be subject to U.S. tax on their worldwide income for any of the 10 years following expatriation in which they are present in the U.S. for more than 30 days, or 60 days in the case of individuals working in the U.S. for an unrelated employer.
IIRC, this law was a result of Ted Arrison giving up his US citizenship very shortly before death, saving a few billions for his heirs.
The law was hastily passed to discourage copycats while working on the exit tax law without haste.