"Between those two savegames we think we've found the bug" (from here)I would presume that if it was a simple solution, it would have been fixed immediately.
Since it hasn't been fixed immediately, I would think there is something complicated causing it to be difficult to prevent.
That was a month ago.
I've even solved it crudely in Project Balance by simply disabling retinues for patricians. It surely can't be that hard to fix when the exact cause is known (a retinue owner dying without an heir).