You’re not alone on this (if you’re talking about the “Human GPS” and “Island Explorer” achievements). Several people have inquired about this exact issue with several other achievements.
Only the Human GPS one hasn’t popped for me right now, I’ve not got round to playing the Storm Island expansion yet, so I’ll look forward to also not getting that achievement there.
It’s because you haven’t driven the 1% of road somewhere on the map. Keep playing the game or just drive around the map and it will unlock suddenly. This happened to me as well and as I free-drove, as I like to do in these open-world games, it unlocked suddenly as I traversed the unknown section of road that was the missing 1%. So just keep driving around the map and it will unlock. This is not a bug per se.
As if by magic, popped it during a bucket list along the motorway. We could really do with something that’ll highlight the final bit of road to avoid headaches from trying to find the missing bit of road haha.
I have discovered this with every Horizon game that even if you drive down every road and the game says so, you wont get the achievement if it is done partially or completely offline. The game will say 100% but the achievement will only say something other than 100% (99% for example)