Steps to reproduce:

1. Go to Classic View.

2. Click on a Task List.

3. Now click on the first Task in the Task List. You will jump to a screen with that task, but the other tasks that should also show up on the left disappear (this is the bug).

4. Now select a different Task List from the dropdown selector on the upper left. Once you select a different task list you'll see all the tasks as you should have on step 3.

Thanks