It looks like the PanelOnly option works for some of the simpler panels but not the more complex ones.
Another thing that you can do as a work around, while we look into the underlying cause, is to create a transparent panel at the top of the screen. The transparent panel forces everything behind it to load.
If you go to your panel.cfg in the Prepar3D/SimObjects/Airplanes/(Your Aircraft)/Panel Folder and add the following items.
Under [Window Titles]
Where N is the next number in configuration and Panel name can be whatever you like. I call the panel "Loader".
Then add this to the end of the document with the N matching the number above.
window_size=0.1 //percent of screen size
position=8 //Position of loading
BACKGROUND_COLOR=0,0,0 //Background color (0,0,0 is transparent)
VISIBLE=0 //1= initially visible, 0= not initially visible
ident=Panel Name //Panel Identification name
zorder=100 //Forces the panel to load on the very top
The last step is to then put this 'Loader' panel on the same monitor as the panel you want to show. I would suggest making the panel some other color, besides transparent, so that you are able to move it to the correct monitor. Once there change the panel to transparent and save your flight.