Then it changes covers based on your last unread volume. If you read the entire series it shows the latest volume cover. This is purely aesthetic, I know.
In the past this has been suggested, but the option that was considered it was simply the first or last volume cover, not based on reading progression...
Add a toggle: on, it displays the last uploaded cover (or some other metric for "last"); off, it displays the first uploaded cover (or some other metric for "first").