i am not an expert on Plasma… but I know that it is not easy to move that taskbar to another display. It could be that also the internal display is disabled the taskbar is still bound to that display.
So it could help to create a new taskbar in the external display.
I tried to edit plasma-org.kde.plasma.desktop-appletsrc according to this video by changing field: lastScreen=1 to lastScreen=0 didnt really fix the issue.
yes do not ask me about the config files on plasma they are really cluttered all over the place, special for someone used to i3 where almost all settings are in one file
You should try to create a new bar on the external display… Or is it set to expand ? i mean there are two ways to add external display… they can mirror the default or expand and stay on its own config.