I’m gonna weigh in on this as I also had this issue. The world missions themselves…imagine as a book with chapters.
You have will have to complete each chapter associated with the world mission - in order for the world mission to count as 1 completed mission to the tracker.
Now this was pre August update, and may have changed but I know for sure this was the issue. To pull it another way I was tracking 20 World Missions complete, and had 6 side missions remaining to do.
4 / 6 were linked to individual world missions, that added to the total count of 24 / 25 once completed. The remaining 2 were linked to the same world mission adding the final count to 25 unlocking the achievement.
I found the issue developed when I progressed further than the game that I think it expected me to do, leaving early world missions in limbo, while I progressed with new ones.
I would recommend going through the log, and looking at what’s left to do…also you can navigate the map and see what missions are left to be found, and specifically work on them. I’d also keep a manual count from your current point to make sure it’s tracking correctly.
Good luck sir