With multi-monitor setups, you effectively need to recreate your panel each time you connect a new display. This can become quite tedious. I propose that the configuration menu for panels includes an option to set the currently selected panel to show on all displays. This is useful for panels used in a similar fashion to the taskbar on Windows. Obviously, there should be a way to override this for some monitors, but I am not sure how it should be implemented. A similar issue was filed for KDE Plasma 4 here [https://bugs.kde.org/show_bug.cgi?id=281463], but was shelved due to KDE Plasma 4 reaching EOL.
*** This bug has been marked as a duplicate of bug 446654 ***