In Pocket Outlook, in the monthly view, all appointments which have a "free" busy status are not displayed. When an "all day" event is manually created in Lightning/Sunbird, it automatically has its busy status set to "Free" (instead of "Busy"). This status can be accessed in the event menu (Options/Show time as). This choice seems to be "by design". By now, the workaround is to change the status to "Busy" when you create an "all day" event in Lightning.