That's probably right it must be a bug as I have played both FH3 and Motorsport 7 as well...let's see if it changes when the game officially launches there are a few nicks and ticks here and there maybe there's a day one patch in the worksProbably a bug since all Forza games since Horizon 3 were Play Anywhere so all progress/achievements are tracked across Xbox and PC.
I got these 3 for Horizon 3, Horizon 4 and Motorsport 7 (all from PC).