All scheduled email reminders (except the first) are ignored

Note that the relationship between civicrm_action_mapping and civicrm_action_schedule is 1-to-many; this observation is supported by a few points:

1. The civicrm_action_schedule.mapping_id is FK referencing civicrm_action_mapping.id
2. There is only one row in civicrm_action_mapping, and it's initialized during install/upgrade
3. The UI (CRM/Admin/*/ScheduleReminders.php) allows end-users to create several rows in civicrm_action_schedule, and they all reference the same civicrm_action_mapping.

However, there are a few spots in the code which appear to assume that civicrm_action_mapping.id = civicrm_action_schedule.id, which is only true for the first schedule.