Someone answered a question I posted, but their answer did not appear when I clicked on the mailbox (inbox tray) icon. (Hovering over that icon gives the pop-up message "recent inbox messages") However, it did appear when I then clicked on the link at the bottom of the drop-down labeled "Go to full inbox".
You can see the issue at "3 hours" in this screenshot
Note that a second answer, (25 minutes in the screenshot) did show up.
I don't think it is asking too much to be notified of every answer to a question I posted by each answer being added to my "recent inbox messages" list. Therefore, I am tagging this as a "bug", and hopefully it will be addressed.