Losing titles on death in administrative Byzantine (help please)

[Mind Eye (6th senz)]

So I have had a successful career for three generations in Byzantine Empire, gaining land and influence with every successor. On my current character, my son stands to inherit what is now the Catapanate of Hellenia as main title. He is also first in line of succession for every title I hold, which includes duchy of Latium, (Rome, Tivoli) Tuscany, Thessaloniki and parts of Georgia.
HOWEVER. When I die, the Byzantine Emperor takes Latium, where my Capital of Rome is. On the death screen, it says that the duchy of Latium is among the titles I have inherited, but when I play, it has been lost and taken over by the Emperor.
My son is my favorite child, designated heir, first in succession order by a vast margin, preferred by all vassal types, perfect governor efficiency, everything. Why is this happening?

TL;DR: On death the emperor takes my land even though death screen says I inherited it. Why?