If a user schedules a Flash Report to run on the 31st of each month, does it only run on months that have 31 days? Or does the report run on the last day of each month by default?
The Flash Report only runs on months that have 31 days and this is working as designed. The recommended workaround is for the user to run the report at 12 AM on the 1st or on the 28th of the month.
Vault Reporting Documentation: Flash Reports