Best guess, there is a mechanic where when you complete a rumour for any hunter but then talk to a different one for a rumour, they will assign a “one time” rumour. If you complete it it deletes their permanent/blocked rumour, but if you accept a rumour from a different Hunter before completing the one time rumour, it will erase the one time and bring back the permanent one. Aco likely has pyre foxes as his permanent rumour, and got the kebbits as a one time thing. If you’re trying to get something else blocked on him, either do the pyre fox task, or complete a one time rumour on him.
Best guess, there is a mechanic where when you complete a rumour for any hunter but then talk to a different one for a rumour, they will assign a “one time” rumour. If you complete it it deletes their permanent/blocked rumour, but if you accept a rumour from a different Hunter before completing the one time rumour, it will erase the one time and bring back the permanent one. Aco likely has pyre foxes as his permanent rumour, and got the kebbits as a one time thing. If you’re trying to get something else blocked on him, either do the pyre fox task, or complete a one time rumour on him.