10 times out of 10 if I right click on the main monitor desktop to bring up properties and select the "Screen Saver" tab, the DF taskbar disappears. I was checking to see if starting the screen saver was the problem, but I don't even get that far and the taskbar disappears. I suspect it's also going missing when the screen saver activates on it's own (which is when I tried testing it). Reboot with same problem.
If I toggle the "Multi-Monitor Taskbar" twice from the systray icon, it comes back but leaves again when I repeat the above (only when I select the "Screen Saver" tab).
This was due to a change with the full-screen detection code, related to games. Unfortunately, when the Display properties window is open it hosts the screensaver in that small frame and DisplayFusion gets a bit confused. I have released a small update to fix this, and you can find it here:
DisplayFusion doesn't try to change the wallpaper when the screensaver is on, although you can control that by changing the "don't check for wallpaper changes while a fullscreen application is running" setting.