The mail system got a slight "upgrade" this afternoon.
First off, it's now paginated as per your listing preferences under settings to make things more manageable for those users with long conversations.
It should jump to the page, and to the message, of the first unread piece of mail you have if you have any unread. If there aren't any unread, it should jump to the last page of your conversation.
If you encounter any issues, it's because I broke something and you can let me know here or in a mail and I'll take a look into it.
Future reworks will be removal of messages permanently so you never see them again, rather than just hiding the conversation from your inbox. And we'll probably end up with a "mails older than XX days get deleted" automatically to keep things under control in the long run. But for now, let's stick with the pagination and auto-jumping to unread and make sure that's okay!