The primary reason the game fails to load a "100% save" or gets stuck is a mismatch between the the save file was created for and the SteamID currently used by the CODEX emulator. If these don't match, the game treats the save as corrupted or belonging to another user, leading to an infinite loading loop. How to Fix the CODEX Save Game Loading
Change the to a custom number if the default one is causing a conflict. forza horizon 5 codex save game 100 fix