Not 100% sure why I popped this just now, but here's what happened:
My tracker for this was 80% complete, and I decided to start a new game with the Italian national team. Only selected the Italian and Icelandic leagues, because I wanted to play the European Championships of 2021. Anyway, I went on to my first game against Bulgaria and easily beat them when this cheevo popped, and got a news item about Italy being unbeaten in 35 games.
So I dunno if it was tracking my other unbeaten games from the different save (I assume not), or just Italy's unbeaten record before I even started the game as their manager.
TL;DR: Begin a new game as Italy's manager in early 2021 (you might need to have the Icelandic league selected as the starting one to get to choose that time period), win the first World Cup qualifier game, and this might/should pop.