I have made the following short script:
let cals = await Calendar.forEvents()
let rems = await Calendar.forReminders()
console.log(cals.length)
console.log(rems.length)
When I run it on my iPad, I get the correct numbers but when I run the same script on my iPhone, the number of reminders is 0. Both runs latest iOS (phone is 12.1.4), it is an iPhone 8. The scriptable version is 1.2.3.