ISISK and IEA (Afghan Taliban and ally militants) have become enemies since Nato withdrawal. TTP is full of pro Taliban groups and some pro ISISK groups but they're not allies with ISISK, they are allies of IEA. Taliban fears if they attack TTP and make enemy of them, they will then join ISISK and harm Taliban interests.
Currently Taliban is facing big war with ISISK, the issue is serious since Saudi Arabia has banned Talighi Jamaat, declared it as a gateway to terrorism, Taliban is return banned Salafism, declared Hanafi fiqh as state law with rights to shia minority, they claim salafism breeds ISIS etc. ISISK also attacked many pro Taliban organisations in Pakistan such as JUIF.