When the first lease payment date is prior to 1970, the generation of the schedules will cause records to be generated well beyond the number of expected records as well as creating duplicates.
As a temporary fix, create individual payment records for those dates prior to 1970. Note that this can be quite cumbersome if the legacy lease dates are significantly further back in time than 1970. We needed to check if the date is the smallest allowed in SQL Server, “01/01/1753”. Oracle and DB2 allow dates as small as 01/01/0001. Moving forward, we fixed a long-standing issue when creating leases with dates prior to 1970.