So, it's intentional. Furthermore, it is historically accurate. Some other EU4 colleagues have very wisely pointed out that Mamluks monarchs don't "die", but rather get replaced. While they do die, in this gov system the system itself is important, not much the Sultan holding supremacy at a certain time. It was a certain caste dominating system, in which slave-warriors (the Mamluks) dominated the Egyptian government. So, as long as there is another Mamluk succesor, there shouldn't be any stability loss, since the system is still working and there's no reason to be less stable. Another slave-soldier replaces the latter one, continuing the Mamluk supremacy over Egypt. I'd say the governing system gets even more stable after each succession, not less stable. A normal monarchy, in which the ruler's personality/family intrigues played a bigger role, can have a stability loss when the current ruler dies. But not the Mamluk system.