Do you have the Alt+Tab Handler enabled on the DisplayFusion Settings > Alt+Tab tab? If not, it's likely the Windows bug where when a button is removed from the Windows taskbar (e.g. if DisplayFusion Taskbar Mode is set to "All taskbars show relevant windows"), it gets unregistered from Alt+Tab/Task View/Virtual Desktops.
There are three possible workarounds:
Set the Taskbar Mode to "Windows taskbar shows all, DisplayFusion shows relevant"
Enable the DisplayFusion Alt+Tab Handler
Disable the DisplayFusion multi-monitor taskbars and use the Windows 10 multi-monitor taskbars
