Application icon is shown instead of the window icon when icons aren't grouped

If you have only one window open of a certain application, it will show the application icon instead of the window icon on the taskbar. This is probably easiest to notice with Explorer. If you have only one Explorer window open, it'll show the default Explorer icon (a closed folder in a blue clip of sorts), instead of the window's icon (eg. a computer icon for This PC, downwards arrow for Downloads, picture frame for Pictures, etc.) on the taskbar. If you have two windows of the same application open*, then it'll show the window icon properly. For example, if I have both This PC and Downloads open, it'll show the computer and downwards arrow icon.

* It will only show the window icon properly on multiple windows if they are grouped. If they aren't, all icons on the taskbar will have the default application icon.

I believe it should show the window icon regardless of whether multiple windows are open or not. It should only show the application icon if the application is pinned to the taskbar and no windows of it are open.

I run Windows 10 Pro x64. Screenshots: