Is it a bug that the previous month’s week is filled-in in the solution when the previous month’s week contains unset shifts ?

No, it is not a bug. The displayed solution is determined by the constraint range, which is from the constraint StartDisplayDate to FinishDate. The solution within the constraint range that satisfies the constraint is displayed, which is the specification.

Even if the last month’s part is not filled with schedules, it is still within the scope of the calculation range, and the specification is to show a solution that satisfies the constraint.

The last month’s part is also subject to constraints. For example, if a six consecutive workday is prohibited, the constraint cannot be satisfied at the beginning of this month unless the last month’s workday is counted from the last month’s workday.

Usually, the previous month’s part is a past schedule that has passed and is usually FILL IN. Still, even if it is not FILL IN, it works the same way as the current month part to generate a solution that satisfies the six consecutive workday prohibitions. This is because, as I said at the beginning, the solution that meets the constraint is displayed within the constraint range, which is the specification.