I've seen this in both Windows 8.1 and 10. Some programs get stuck behind the taskbar, like VLC, Spotify, Opera, and Powershell.
I want to have the taskbar at the upper edge of the screen, and it's not set to hide automatically.
Screenshot of Powershell and Spotify stuck behind the taskbar.
Just tried it here with the task bar at top and no auto-hide. Power Shell does get it's title bar up under the task bar when you open it, but as soon as you resize it even a little it pops out from underneath. Definitely a bug that should be reported via the Feedback app.