Every new user considers this behavior wrong, as it is so counter-intuitive. However, this is the intentional Asana structure…subtasks do NOT inherit the Project of their parent Task. You can deliberately move any Subtask to the Project of its parent Task…and then you’ll see lots of duplicates in Timeline and Calendar.
See workarounds F2 and F3 here: 🔥 Hottest feature requests and their workarounds