VisualEditor (NWE) doesn't exit cleanly when clicking the namespace tab on a discussion page.
Ok, so I managed to pin down why the editor save box doesn't show up sometimes. When editing a non-talk page, clicking the tab for that page triggers the editor save box (e.g. "read" tab on an article, "project page" tab on project namespace, etc.), but clicking anything that would change the URL (talk page, main page, your user page, etc.) triggers the browser's unsaved changes prompt (so no ability to discard).
Conversely, when editing a talk page, clicking the talk page tab doesn't trigger the editor save box, but the browser's unsaved changes prompt, just like clicking away would. Maybe this should be changed so clicking the talk page tab works the same as the main tab does when editing.