Slitherine

Playing as Aevinwode, Sonneneve are great pals with me for most of the game, but then suddenly go neutral and even to -3 diplomatic relations with me (though still allied so far) and this when i'm handing them provinces all the time and giving them resources they need (horses, gems). I look up their diplomacy screen and it gives a big minus for "annexed province" (-37.5), but i've never ever annexed any of Sonneneve's provinces at all. I've never even been to war with it and hand it over pretty much every province i take from other kingdoms that's on its borders.

Possible bug.

Also is it possible that you own one of Sonneneve's original provinces?

no, definitely not - i've played as Sonneneve and as Aevinwode so i know which are the starting provinces for each. It shows the same thing for lots of kingdoms you don't even have contact with , pretty much whoever you play.