To simplify Stahps guide to fixing this achievement this is all you have to do in a much easier way.
If you are stuck at 95% like I was after unlocking every skill you will need to follow these steps.
•Disconnect your wifi or Ethernet cable from your Xbox (Do Not select go offline if you are using wifi)
•Go to Forza Horizon 3s Manage game and delete your save file.
•Start a new game. Unlock the skill points I needed. (To Be sure I unlocked 7 skills as man the skills)
•Pause the game and dashboard to the Xbox main menu. Go to manage game and delete the hard save file again. (DO NOT QUIT THE GAME)
•With Forza Horzion still running and the save file deleted, go back to your settings and connect the wifi. If you are using an Ethernet cable plug I think in now.
• Go open Forza Horizon and the achievement will pop.
This solution also works for Exhibition Expert I had completed every exhibition and the achievement was stuck at 97%. And it unlocked at the same time as I want it all.
Do keep in mind that your game progress may not show the correct stats. Do not worry. Nothing will rest as everything is backed up on your cloud save.