If today is the 5th and there are hourly backups - but not for the time between 24h ago and the end of the 4th, then NO backup will be kept for the 4th. Even if the program works as specified, this behavior is counter-intuitive.
Expected behavior: Keep at least one directory for the 4th.
Check: The same behavior is to be expected on the border of daily to monthly backups for the last month.