Hi Mokerak!
It looks like the situation you're describing is actually working correctly, Tanistry is affecting only the chosen title. So if you choose Tanistry for the Kingdom of Ireland title then it will only affect the kingdom title and not any other titles you may have owned. Those will still be affected by your realm's Succession Law, you can check it in Inheritance tab in the Realm View. Thank you for your report nonetheless!
It looks like the situation you're describing is actually working correctly, Tanistry is affecting only the chosen title. So if you choose Tanistry for the Kingdom of Ireland title then it will only affect the kingdom title and not any other titles you may have owned. Those will still be affected by your realm's Succession Law, you can check it in Inheritance tab in the Realm View. Thank you for your report nonetheless!