In 2Do for Mac, sub-tasks under a project will always be displayed - no matter which filter is currently in play (focus filter, smart filter and so on). The reasoning is pretty simple: when you collapse a project, your natural expectation is to immediately see all your sub-tasks. Applying focus filters and smart filters at a project level would needlessly complicate the UI and confuse users. For instance, if 2Do was to hide scheduled sub-tasks under a project, the user may assume the app’s deleted them or the tasks have gone missing (since there would be no obvious way to toggle their visibility).

Instead, 2Do displays sub-tasks separately on the main task list if it needs further action. Have a look at the example below with the Today list selected. Here, we have a project called Loft Extension, with a due date of Today. Under it, one of its sub-tasks also shares the same due-date. As you can see, collapsing the project will always display the sub-tasks, however in this situation 2Do will list the sub-task "Pay John advance and confirm start date" on the main task list as well. To make the matter clear, 2Do also displays a little project icon with the project title below the sub-task, indicating that the sub-task belongs to the mentioned project. 

Displaying sub-tasks this way on the main list actually helps you focus on what’s important, without worrying about missing out on those sub-tasks with due dates deep inside of a project.