When the taskbar is not on the bottom, then the fullscreen mode does not work. I have the very same problem. My taskbar is on the left, and if I set an app to fullscreen, it does not cover the taskbar. If the taskbar is on bottom, it works. It is a bug in Windows 10 and it's been there since the first version.There is a bug entry already in the feedback app, but only 3 people reported having it?!?!
So if you also have this problem, you should report this. The bug entry is in the taskbar category.
I really hope more people will report this, to get this fixed soon.