Ok, after reading a few threads and doing a bit of tinkering, I have come to the conclusion that the permanent 15% or 30% Unrest is a particular feature of that unique region to indicate its rebelliousness. I remember this was the case for Scotland in M:TW as well as a few other Spanish provinces so it's not a complete surprise.

Regions which end up with a permanent 15% Unrest, initially start with 45% Unrest when first conquered (that permanent 15% doesn't decay with time though the other 30% does as usual).

Regions which end up with a permanent 30% Unrest, initially start with 60% Unrest when first conquered (that permanent 30% doesn't decay with time though the other 30% does as usual).

I haven't conquered every region in my games, so I will need the help of forum members here to compile a list of towns which have this particular "rebellious" feature to indicate permanent Unrest. Kindly check the list to see if the data here matches the data from your games.

Towns which have permanent 15% Unrest:
Segestica
Salona
Domus Dulcis Domus
Vicus Gothi
Vicus Marcomanii
Mogontiacum
Batavodurum
Damme
Osca
Carthago Nova
Corduba
Tylis
Tanais
Croton

Towns which have permanent 30% Unrest:
Londinium
Tarsus

Towns which have no permanent Unrest:
Porrolissum, Vicus Venedae, Campus Getae, Campus Scythii, Byzantium, Campus Sarmatae, Condate Redonum, Narbo Martius, Aquincum, Lovosice, Iuvavum, Lepcis Magna, Patavium, Mediolanium, Segesta, Trier, Caralis, Massilia, Lugdunum, Alesia, Lemonum, Thessalonica, Bylazora, Campus Iazyges, Cyrene, Capua, Carthage, Thapsus, Lilybaeum, Messana, Tarentum, Apollonia, Larissa, Nicomedia, Ancyra, Sardis, Sinope, Mazaka, Kydonia, Rhodes, Corinth, Athens, Antioch