After further observation, this behavior appears to occur for "Archived" items.
For example, you can select a milestone and all of its tasks on the "Tasks" pane, then click the "Archive" button. All the tasks are "moved" to "Archived objects", and disappear from the Tasks pane. The Milestone, instead, is not (ie, it does *not* show up in the Archived objects pane, and does remain in the Tasks pane).
I couldn't tell whether this is a choice or a little bug (milestones not being archived), but it is a bit confusing to have very old milestones appearing there, empty, along with the new ones. Also, "+ Add task" shows up for them, and you can successfully enter tasks clicking there. However, such tasks are then displayed in the "Unassigned" category. Subjectively, I'd say this is a bit confusing and moving the Mileston too to the archive would be best.