Forza Horizon 5 has just received a patch that fixes issues with two of the new achievements introduced in the Series 10 title update, making them unlockable and the full completion possible.The two offending Forza Horizon 5 achievements — One for All and Back in the Saddle — simply ask that you complete a Horizon Open Custom Race in two specific vehicles, and even though the in-game Accolades for completing these tasks was popping just fine, the related achievements were not. As of today's update, this should have been remedied, and we've already started seeing some unlocks from tracked players here on TA.
Forza Horizon 5 broken achievements fixed
If you've already got the Accolades, you should just need to update and launch the game to receive the newly-fixed related achievements. Fortunately, if they don't decide to pop for you (or you haven't completed them yet), they're basically just participation medals for those couple of Horizon Open races, so it shouldn't take long to meet the requirements again!