I'm using a second monitor in a dual head setup.
I am using xrandr to put the second monitor to the right of the main one.
The problem is: if I have another task list on my top bar(plus the one on the bottom bar), and I move a window on the second monitor, its task button disappears from both the task lists.
If I move back the window to the main monitor, its task button reappears. 

The normal behaviour (i.e. the task buttons do not disappear) is followed only if I remove my top bar's task list.