Just to want to say I’m also struggling with this! I believe they said originally that it was only changeable in the desktop or Mac version. But as an iOS user, I’m unable to cancel current tasks! Annoying!
I have the same issue. A task that I created in the early release phase of “tasks” keeps sending me notifications but does not show up in the “Tasks” (chatgpt dot com/tasks) page where I can control other tasks. This zombie task causes me to turn off all notifications for gpt.