When I duplicate a task in the web app, the task along with all of its subtasks is duplicated. But, when I duplicate a task on my Android phone, only the task is duplicated empty (without all of the subtasks). Is this a bug or is there something I don't understand about the duplicate feature?

Seems like a bug to me. TickTick is consistent when it comes to having cross-platform features, just in different places depending on the type of device.