If I remember correctly, the preview will use correct sunrise / sunset time for today’s schedule, but if you check any other day, it will use the default sunrise / sunset (6:00 and 18:00) as the controller only tracks and reports today’s sunrise / sunset times and not other days.
You are right that the sunrise / sunset times are obtained through the weather call — if your diagnosis shows that the controller is not getting successful weather call, then it won’t obtain the correct sunrise / sunset times.