为何添加软约束后OptaPlanner无法找到可行解？

问题背景 #

项目原本包含以下两个硬约束：

fun cpMustUseN(constraintFactory: ConstraintFactory): Constraint {
        return constraintFactory.forEach(MealMenu::class.java)
            .join(CpMustUse::class.java, equal({ mm -> mm.slottedCp!!.id }, CpMustUse::cpId))
            .groupBy({ _, cpMustUse -> cpMustUse.numRequired }, countBi())
            .filter { numRequired, count -> count < numRequired }
            .penalize(HardSoftScore.ONE_HARD)
            .asConstraint("cpMustUseN")
    }

    fun cpMustUseAtLeastOne(constraintFactory: ConstraintFactory): Constraint {
        return constraintFactory.forEach(CpMustUse::class.java)
            .ifNotExists(MealMenu::class.java, equal({ cpMustUse -> cpMustUse.cpId }, { mealMenu -> mealMenu.slottedCp!!.id }))
            .penalize(HardSoftScore.ONE_HARD)
            .asConstraint("cpMustUseAny")
    }

运行相关测试用例时，OptaPlanner能够找到得分为0hard/0soft的可行解。

但添加以下第三个软约束后，系统无法再找到可行解，测试用例的最优结果仅为-1hard/-3soft：

fun cpVariety(constraintFactory: ConstraintFactory): Constraint {
        return constraintFactory.forEachUniquePair(
            MealMenu::class.java,
            equal(MealMenu::slottedCp)
        )
            .penalize(HardSoftScore.ONE_SOFT)
            .asConstraint("cpVariety")
    }

根据OptaPlanner文档，只要存在可行解（无硬约束违反），无论软约束违反情况如何，都应优先选择可行解。确认当前场景存在可行解，但系统未找到，请问可能的原因是什么？

编辑补充：最终可用约束 #

为方便后续读者，以下是基于Lukas的回答调整后的最终可用约束：

fun cpMustUseN(constraintFactory: ConstraintFactory): Constraint {
        return constraintFactory.forEach(MealMenu::class.java)
            .join(CpMustUse::class.java, equal({ mm -> mm.slottedCp!!.id }, CpMustUse::cpId))
            .groupBy({ _, cpMustUse -> cpMustUse.numRequired }, countBi())
            .filter { numRequired, count -> count < numRequired }
            .penalize(HardSoftScore.ONE_HARD) { numRequired, count -> numRequired - count }
            .asConstraint("cpMustUseN")
    }

    fun cpMustUseAtLeastOne(constraintFactory: ConstraintFactory): Constraint {
        return constraintFactory.forEach(CpMustUse::class.java)
            .ifNotExists(MealMenu::class.java, equal({ cpMustUse -> cpMustUse.cpId }, { mealMenu -> mealMenu.slottedCp!!.id }))
            .penalize(HardSoftScore.ONE_HARD, CpMustUse::numRequired)
            .asConstraint("cpMustUseAtLeastOne")
    }

    fun cpVariety(constraintFactory: ConstraintFactory): Constraint {
        return constraintFactory.forEachUniquePair(
            MealMenu::class.java,
            equal(MealMenu::slottedCp)
        )
            .penalize(HardSoftScore.ONE_SOFT)
            .asConstraint("cpVariety")
    }

内容的提问来源于stack exchange，提问作者Chris