did you put your king in a position where he was seperated from your povinces by quite a few turns ?
thats usually my problem when i get a decently big drop in loyalty levels.
or has your king died ? that can cause a sizeable drop depending on the old king and the new king, his vices and virtues, influence as well. (and where he is on the map, more than a few turns away)
though im not too experienced with single player so theres probably more.
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