A part of the game that I do not understand is how it decides exactly which provinces you can immediately start coring when you take a contiguous block of provinces, not all of which are adjacent to an existing core or exposed to a coastline your coring range can reach.
In my current Ottomans game, with Wallachia intact and unvassalized, I took Temes, Hunyad, Maros, Zemplen, and Varad (not to be confused with Varasd) from Hungary. Of these, the only province directly bordering an existing core is Temes, which borders Vidin. The game allowed me to immediately start coring Temes (obviously), Hunyad (adjacent to Temes and Varad), Varad (adjacent to Temes and Hunyad), and Zemplen (adjacent to Varad), but not Maros (adjacent to Hunyad).
Possibly it is relevant that Maros has the region "Dacia" listed first, followed by "Hungarian Region", while the other four are only in "Hungarian Region".
In my current Ottomans game, with Wallachia intact and unvassalized, I took Temes, Hunyad, Maros, Zemplen, and Varad (not to be confused with Varasd) from Hungary. Of these, the only province directly bordering an existing core is Temes, which borders Vidin. The game allowed me to immediately start coring Temes (obviously), Hunyad (adjacent to Temes and Varad), Varad (adjacent to Temes and Hunyad), and Zemplen (adjacent to Varad), but not Maros (adjacent to Hunyad).
Possibly it is relevant that Maros has the region "Dacia" listed first, followed by "Hungarian Region", while the other four are only in "Hungarian Region".