Same issue exists in version 1.15, probably due to the fact that the taskbar button is maintained by Windows now.
The application button is only visible on the main screen task bar, even though the app is positioned on a secondary screen.
OS: Win 8.1 Pro Update 1
Yes, the taskbar button is now under the control of the OS now on 8.1 Update 1.
Unfortunately that means any issues with that would need to be resolved by Microsoft.
There is a registry key to switch ModernMix back into providing the buttons, but it has other side effects and we would not recommend the use of that,