There is data left from GO sent to the server, on the server side.
While we cannot access it normally, it’s definitely there. The easiest example would be to send a mon from GO to HOME, and view the data on mobile. It tells you who the GO trainer is, and that info isn’t tied the PK data, but rather the HOME tracker.
While it doesn’t show when traded to a different HOME trainer, it shows back up when traded back.
(anyhow, even if it eventually disappears, that just the UI doesn’t show them. The info is likely still on their server)