We had a five page thread; Spotify closed it because "nobody responded in a long time". That's because, Spotify, ya never fixed it.
How to reproduce (you need two monitors):
1. Open Spotify and close it on the second monitor.
2. Open Spotify again. The icon will be on the primary monitor, but the window will be on the secondary monitor.
Windows 10 Pro x64, version 1803 (April 2018 update).
Spotify version 126.96.36.199.g936eab8d (Windows Store version).
See this 45 second video for the full walkthrough (see a normal application vs Spotify). Put the icon on the correct taskbar, Spotify.
// IGNORE THE TOP ANSWER BELOW: THERE IS NO ANSWER NOR SOLUTION. SPOTIFY COMMUNITY MODS BARELY UNDERSTAND WINDOWS.
// IGNORE "Top Answer" BELOW: