Luna display and 5k iMac

I connected my 2018 12.9 iPad Pro to my late 2014 27” 5K iMac and Luna Display installed perfectly.
When I choose to enable Retina display, the iPad shows to iMac desktop and fills the screen instead of having the two black bars across the top and bottom as with just Astropad Studio alone.
However, the iMac screen shrinks to about 50% of the screen and the rest is black.
Is that normal operation or do I have a defective unit? I can find no info on your website about running Luna Display on a 5k iMac.
As a matter of fact, there is no information anywhere (Youtube, Google, etc) about the Luna Display and 5K iMac combo.