I found the same thing happening quite frequently for me over the last couple of weeks.
I was able to contact one of the players and apparently they were somehow still playing the old version of the island (and I guess adding metrics to the private island code, rather than the current release).
It would seem that when they opened Fortnite, the island was still selected as their game mode from the last time they had played; the catch being that it was the old version of the island, as a new version had since been released.
Until they manually opened the game selection menu and re-selected the game, they were able to play in the previous released version of the island.
I haven’t had the chance to verify this myself yet - but it would explain the issue if that was actually the case.