If you force defender to transfer the province to someone, the defender looses core, even if the province is your core and your culture.
If you force defender to release nation, defender gets to keep the cores, even if released nation is of different culture, and then defender can use reconquest CB to quickly kill off released nation if they lost alliances.
Why the inconsictency, PI?
If you force defender to release nation, defender gets to keep the cores, even if released nation is of different culture, and then defender can use reconquest CB to quickly kill off released nation if they lost alliances.
Why the inconsictency, PI?