As David C noted, the problem with many of the junior GSs is that you get quite a few young players coming in (as WCs or DAs) from countries where they don't put so much emphasis on ITFs, and they're very underranked in ITF terms. Korda won or was runner-up in one of the US nationals, I seem to remember. From France, Corentin Moutet is another - has played three ITFs this year: lost in finals of the European Junior Championships to Tsitsipas, lost at the French to Blancaneaux for two of them, and I think Mahut recently tweeted that he'd won the French nationals. But not seeded here.