Date Pattern  '.'yyyy-MM-dd-a (HALF_DAY) is not treated as Valid PeriodicityType[0]
So RollingCalendar.computePeriodicityType() - return TOP_OF_DAY as periodicity.[1]
Which causes rollover at midnight instead of twice of the day (at midday and midnight)
 

[0]: https://github.com/qos-ch/logback/blob/bbc783e6d68482abe8e7dff0d06708175e971b91/logback-core/src/main/java/ch/qos/logback/core/rolling/helper/PeriodicityType.java#L22 

[1]: https://github.com/qos-ch/logback/blob/bbc783e6d68482abe8e7dff0d06708175e971b91/logback-core/src/main/java/ch/qos/logback/core/rolling/helper/RollingCalendar.java#L52