Yeah, this is definitely caused by disabled notables with < 100 power. For dead notables they already have a replacement system. When a notable dies they get replaced by a relative and all of their relations transfer over seamlessly, so disappearing notables is not caused by death.
The chance for notables to be disabled has been recently lowered too. The equation is now (((100 - Power) / 100) x 2), so a notable with 50 power has a 1% chance of being disabled per day (if they have no caravans, workshops, or property). There is also actually a system that is supposed to spawn new notables each week if there are any that are missing in a settlement, but it's not triggered by the disabling procedure as of right now.
Hopefully that all makes sense.