Hi @Alex_Bailey1 
and thank you for taking time to report this.
As @Phil_Seeman explained, this is an intentional behaviour. When the Subtask is associated to a Project, it will automatically appear in that Project’s list of task. To avoid confusion, I’d recommend creating a Section call subtasks and nest all your subtasks under this section so they don’t bother you in your tasks list view.
Allowing subtasks to automatically inherit their parent task project it’s a popular request in the Forum, and this feature is definitely on our radar. And while I don’t have a precise timeframe to share yet, I’ll make sure to post any update in the following thread: Allow to automatically add subtasks to project. If you haven’t yet, I encourage you to add your vote and comment.
Hope this helps! Have a great Tuesday!