Welcome to the community. You are not doing anything wrong except that entering a task list in an update is something totally different for adding subitems. The tasklist in an update is widely used as a very simple tick-off list. You will notie a small circle icon next to the subitem icon. When you finish a few tasks as done the circle will become partly green.
Subitems are most of the time used for real subtasks, with an owner, a status, a timeline and whatever column you want to add. A subitem is basically the same as an item but it “lives” under the main item.