Got it. Honestly I like KDE, it's highly customizable which is a blessing and a curse.
There is not really a concept of a "primary" task bar. You customize each task bar on each screen individually. My guess is that when you disable a monitor, KDE goes "oh, new setup configuration, let me reset myself to nothing". So, if you add a task bar to your monitor where it is missing, it should remember you settings for each monitor state.
There are a whole host of customization you can do on the taskbars in KDE, but it does take a bit of fiddling to make it work how you want.