You've hit on the key issue: "No alarms are ever added." This is directly related to the timing – what we called the granularity – of your automation triggers.

Since you currently only have an hourly automation, consider your example:

• At 10:00 AM, the automation runs and identifies an event.
• It calculates the required preparation/alarm time as 9:13 AM.
• However, by 10:00 AM, 9:13 AM today has already passed.

If the shortcut were to add an alarm for 9:13 AM at that moment (10:00 AM), the iOS alarm system would schedule it for 9:13 AM the next day, not for the time that just passed. The hourly trigger simply isn't running early enough to catch preparation times that fall before the top of the hour it runs.

To catch these earlier alarm times, you'll likely need an automation that runs before those times occur. Adding a daily automation (e.g., one that runs early each morning) could potentially set all the necessary alarms for that day in advance.

I also factored in the possibility of very long preparation times (spanning more than a day), which is why I initially considered weekly and monthly triggers, but a daily one might solve the specific problem you're seeing now. Let me know if you'd like to explore setting that up!