I'm not sure if this is a bug or the intended behavior, but it was reported to me by a user on our install. The user is using subprojects as a way to tag tasks within their project. Let's call the parent project X and the subprojects A, B and C. The user added subproject A to a task, which removed project X. The user was expecting to be able to see the task on the parent project workboard, but could not.