Does (AppServiceTriggerDetails)args.TaskInstance.TriggerDetails work?

No, I get an InvalidCastException in this case.

Can you try to check if the type of TriggerDetails is WinRT.IInspectable?

Using GetType().FullName indeed returns WinRT.IInspectable when publishing the msix with AOT and AppServiceTriggerDetails otherwise.

I've found the issue with the as operator. Since C#/WinRT could not reason what the actual type is in AOT mode (since it doesn't support dynamic code), it fallbacks to the IDynamicInterfaceCastable implementation. That implementation then checks the helper type of AppServiceTriggerDetails, which it expects an interface. However, the helper type of AppServiceTriggerDetails is a struct, not an interface, hence it fails.

I haven't managed to see why As<TInterface>() works though, since from my initial look it seems to also be going through IDynamicInterfaceCastable?

Given this is a class type with just an WinRT exclusive interface, there isn't any IDynamicInterfaceCastable interface today. I assume As<> works because it is making sure the type doesn't get trimmed whereas with just a cast, .NET is deciding it can trim it as it only seen it used with the cast especially given the API that it is returning this says it returns an Object.