1.11.2
When I reform a pagan religion(for example, excalibur bookmark, aelle anglo-saxon religion) and choose feature agnatic clan, the succ law will changed to old feudal, but after that if I adopt germanic government and grant a duchy title to my same religion count vassal, the succ law of the vassal will change to gavelkind(when grant a county, the succ law will first be gavelkind and change to old feudal 1 day later automatically), and will never change back.
And I tried another test, if I do not adopt germanic and stay tribal/high tribal, grant a county the succ law will be old feudal from the beginning, and then grant a duchy will not change the succ law to gavelkind.