This issue has been found to be reoccurring, and we're investigating.
We're currently working on addressing an issue where "Ride On's" given from the Zwift Mobile Link app don't count towards the achievements for giving multiple Ride On's to other users.
If you want to get the achievements now, you can use your computer to mouse over the riders you want to give Ride On's to and those will count towards the achievement. More information is available in this article.
We appreciate your patience and understanding as we get this fixed!