FIX!
When Apple updated to IO12, they deleted 'Show In History' so if you didn't have your app notification set up with the 'Notification Centre' box checked, you'll no longer receive notifications.
But it's not obvious that this needs to happen, and I couldn't find the answer online anywhere (including the Garmin online help centre).
Here's the simple fix: On iPhone, all apps need to have the 'Notification Center' checkbox ticked. To do this, go to Setting>[AppName]>Notifications>Notification Centre (checked).
And it'll work.