How do I force iOS Reminders to sync?
Opening up the calendar and allowing it (or forcing) to sync will do the trick, though oftentimes imperfectly.

To force calendar to sync, open it up and the tap "Calendars" at the bottom, then swipe down.

As far as "imperfectly" goes, I mean it doesn't seem to sync all tasks all the time. I haven't figured out what the pattern is there, but it seems that it's usually recurring tasks that have issues.