Same here, though a workaround is to minimize and maximize the app with missing icon. That makes the icon re-appear. But even that doesn't work if one has switched virtual desktops and uses multiple monitors with one app being shown on the second monitor only. In that case, minimizing and maximizing doesn't restore the icon in the taskbar of the first monitor. Instead, one needs to move the app window to the first monitor, minimize+maximize to restore the icon and move the app-window back to the second monitor.
I'm pretty often surprised how these very basic things get broken in Windows. Nobody of MS's own users seems to use such basic things, even though virtual desktops were introduced as a main feature of Windows 8 or 10.