I guess it is a bug in that from a UI/UX experience, kinda random on sort order of alerts. But in big scheme of things, don't think it's a real issue. Bottom line, does not affect WHEN the alerts go off: one will trigger at day before, the other at hour before.
This also happens on OS X Calendar. I created a new event the other day on my Mac, entered three alerts in the following order: 30 min before, 15 before, 10 before. Calendar on OS X and Fantastical on iOS both show the alerts in ascending order (10, 15, 30). A different event I created a day or two earlier, alerts at 1 hour and 30 minutes before, on OS X Calendar shows 1 hr then 30 min, on Fantastical, 30 min then 1 hr.
I've never paid attention to display order, in that never really cared, and my alerts alert when they are supposed to.
If you have a developer account:
bugreport.apple.com
Bigger UX/UI issue for me is that on OS X, can add more than two alerts, while on iOS, only allows two alerts (hence one reason to use Fantastical or some other Calendar/Reminders app, where 3rd party apps allow entering more than two alerts). Or, on OS X Calendar, can set week start day (I like Sundays), while on Reminders cannot set a day, and the week start for that is Monday (and if I'm not careful, can select the wrong day for an item from the pop-up calendar).
