This is actually not a bug but is part of a new change in Asana where the first level of subtasks now appears in an outline format in a project’s main task list. As part of that change, custom fields have to be added to the subtasks (even though the subtasks are not attached to the project). You can read more about it here (especially my explanatory posts in that thread and add your vote if you’re not happy with the change):
and add your vote if you’re not happy with the change):