To the best of my knowledge MB is doing nothing wrong. Its simply because MB has implemented the API calls needed to show a custom taskbar popup that makes it behave differently to how other apps normally do when using the taskbar preview. If you dont care about the taskbar player showing the playing track and this issue bothers you, in Preferences/ Layout(2)/ taskbar player
untick "show playing track details in the taskbar popup window" and it will behave more normally