This might be related to this bug and the older archived one it references: https://community.spotify.com/t5/Ongoing-Issues/Desktop-Windows-Can-t-access-auto-hidden-taskbar-whe...
On a my Windows 10 laptop, spotify initially prevented the taskbar from showing when I moved the cursor to the bottom of the screen (the expected behavior). This was the case both when the window was maximized and when it was resizable. Moving the cursor to the bottom of the screen changed the cursor to the resizing cursor. By resizing the Spotify window vertically to make it smaller, I was able to access the area that shows the taskbar when you hover over it. Now it works as expected even when Spotify is maximized. Similar things have been known to occur with other apps and other systems, with a variety of resolutions and displays, so I reckon this is a Windows issue... but perhaps the devs can get around it 🙂
tl;dr: if taskbar in Windows 10 is set to auto-hide and doesn't show/stays hidden when you use Spotify, resize the spotify window to make it smaller vertically, then hover over the taskbar to show it. After that, it should be fixed.