Sounds like it's far from automatic. (Well, perhaps "legally" you automatically lose it, but there's no system that actually tells Japan that one of their citizens has gotten another citizenship.)

She became a U.S. citizen in 2004.

After traveling back and forth between her home countries with both nationalities for years, she was flagged as being a dual citizen in 2017 — first at a passport office in Tokyo, where her passport was confiscated and her application rejected, and then by airport immigration officials when they realized she was exiting Japan with an American passport that had no entrance stamp.

So it took them 13 years to figure this out, and if she were actively trying to conceal her US citizenship, could probably have held both indefinitely.