Preferably, these get reported through the official support channels for proper tracking and so I’m not a single point of failure.
There are multiple factors in play here.
Sunrise and Sunset in the Classic app is scheduled by the Weather Station SmartApp which is part of Hello Home and to function properly, a user needs to set a geolocation. Hello Home is installed when a user creates a location in the Classic app or if a user created a location in the new app and then uses the Classic app.
If a user starts out with or only uses the new app, they will still need to set a geolocation. There are warnings and you are unable to create a custom automation or Smart Lighting automation that utilizes sunrise/sunset without setting a geolocation.
However, there is a bug that I was able to reproduce where a user can create a sunrise/sunset automation in Smart Lighting in the new app without having Hello Home and Weather Station. If a user is in this situation, they should be able to sign into the Classic app and Hello Home/Weather Station will install and sunrise/sunset events for Smart Lighting will execute as expected.
If the user manually uninstalled Hello Home on their own in the IDE, using the Classic app won’t re-install those SmartApps and the user should reach out to support or transition to custom automations.
All of this is likely confounded by the confusion over the change in the recent Classic app updates where if a user has removed all of their Routines, they are unable to view the Routines tab. Many users who have claimed they need have Hello Home re-installed actually have Hello Home and Weather Station installed but they have uninstalled all Routines. If the user is insistent on using Routines, the entire Hello Home SmartApp will need to be re-installed by support.
Edit: Another point of failure is if the Weather Station SmartApp fails to reschedule. This can be checked by the user in the IDE and they can update it to get things going again. We used to see this more frequently in the past but a user just ran into it which reminded me.