I can load an old save,
from 829, and the chronicle there goes back to 769. So it appears that something caused it to drop the content. If I load the
save I have from 883 (I don't have any saves from between), the chronicle only goes back to 869. The first entry in the 869 one is on succession, and looking at the my dead characters, I notice that they have the kingdom of Ruthenia as their primary title, even though I always had Poland as my primary. When my character died in 869, Ruthenia (and Bohemia and Lithuania) split from my character due to elective gavelkind. I think something screwy must have happened with that succession, which made the game loose everything from before the succession in the chronicle.
I hope this is of help.