Does anyone know what's the 'opinion' modifier 'have a core'?

[byebytoad]

re'aAs a Milan I forced Aragon to transfer Naple as my Vassal.

Before that in the previous war (about 20 years earlier) I took 4 regions from Naples.

Now after I made Naples my vassal, there's a modifier that affect their opinion negatively by -20.
It's called 'have a core'. You can find it here among the other modifiers.
More than 15 years passed since they became my vassal ,improved their opinion above 100, but I still have this negative modifier.

What I know for sure is that:
-it's not due to me having claims on their regions as they expired few years ago.
-it's not caused by them having cores on their ex-regions.Even if I give them back their previous regions the modifiers won't still go away.

Anyone expert here know what it is and possibly knows how to make it go away?

 

[Kergan]

Naples is unhappy with you because you have one of its cores. Give Naples its cores back, and the modifier will disappear. (It might need a month tick to go away.) Or just ignore the modifier - you can overcome it with the 50 + 200 opinion modifier anyway.

 

[deathbypie]

re'aAs a Milan I forced Aragon to transfer Naple as my Vassal.
-it's not caused by them having cores on their ex-regions.Even if I give them back their previous regions the modifiers won't still go away.

It is caused by this. Select Naples while using the diplomatic map mode, and their non-owned cores will be striped green. You own one of these. It might be one of the provinces of Sicily that Aragon starts the game with, as Naples starts with cores there.