Update: We've identified the cause of this problem as being specific to 32bit iOS devices (i.e. older models) and are working on a fix that should be released soon. Thanks for your patience!
We're currently working on addressing an issue where "Ride On's" given from the iOS version of the Zwift Mobile Link app don't count towards the achievements for giving multiple Ride On's to other users.
We appreciate your patience and understanding as we get this fixed!