A: You likely ignored the Cloud Sync step. Turn off Cloud Sync, delete the save, paste the 100% save, then restart Steam. Alternatively, the 100% save is for a newer/older game version (e.g., your game is v1.6, the save is v1.5).
Delete the contents of 1551360\remote . Do not just paste over; clean it first. forza horizon 5 pc 100 save game
Paste the downloaded 100% save files into the remote folder. A: You likely ignored the Cloud Sync step