I added the running tasks into the Nexus and found all of them except for Google Chrome there. I found out that the Internet icon (globe with yellow orbits) is the placeholder for Chrome. But I also run Microsoft Outlook, and I noticed that not only is there an email icon labelled Microsoft Outlook next to the internet icon, I also have the Microsoft Outlook icon in the running tasks. Since the internet icon took over for Chrome, I would expect the email icon to take over for Outlook. But it doesn't.

Is this a bug? (I'm asking because it's such a minor issue, if it's an issue at all.)

Probably a 64 bit vs. 32 bit issue.

If you are running the 64 bit version of Outlook but the shortcut in the dock is pointing to the 32 bit version (or vice-versa) then they are not considered the same application (because they aren't!) and that happens.

That looks like the problem. And it looks like I won't be able to resolve it fully since Microsoft uses a nonstandard shortcut for the 64-bit Outlook. Oh well, I removed it from the dock. Ka-BOOM!!! There's something deeply satisfying about seeing a Microsoft icon being blown to smithereens...