Since upgrading to Agenda 10 on all my devices I can confirm that:
Using the \remind shortcut or the reminder icon on the iPhone correctly sets up a reminder in Reminders on the phone. This reminder appears siloed on the phone as it does not sync to either Mac or iPad.
The same is true when setting up a reminder on the iPad. It is set up correctly on device, but is not synced to my other devices.
If the \remind shortcut or the reminder contextual menu item is used to set up a reminder on Mac, this is set up correctly on device and syncs correctly to both iPhone and iPad.
I hope that helps. No silver bullet found.