Byzantine succession is broken

[SGMG2020]

In my current playthrough as Byzantium, it’s happened again: the Byzantine succession is broken. I painstakingly built up a ruling dynasty, frequently marrying female family members back into the family—enough to make any Habsburg envious! ;) But then, suddenly, my ruler died, and his wife—who was also a member of the dynasty—became regent. Before the ruler passed away, I had what felt like an endless supply of heirs; then, while he was still alive, I suddenly had none left in the line of succession. I was able to elevate the regent to ruler, and she had three daughters—my family tree was vast and deeply branched—yet, somehow, there was no heir? Now she has passed away, and I’m stuck in an interminable regency. Tinto, what on earth are you doing? Is this stuff not thought through? Do you just toss in game mechanics and wait to see how they play out? I spent centuries building up Byzantium with a single dynasty, and now—pardon my language—this kind of ♥♥♥♥♥♥♥♥ happens!

Can someone please explain what this is all about?