The Blackmagic Design DeckLink Duo 2, DeckLink Quad 2 and DeckLink 8K Pro video devices require correct Connector Mapping Settings, to provide all available inputs (four or eight) to the Just In Engine (Mac) application. Please open the macOS System Preferences and open the Blackmagic Desktop Video Preference Pane.
There you can see your connected Blackmagic Design video devices. Click on the button in the middle of the screen to open the device settings.
Then switch to the Connectors Tab and set the Connector Mapping to SDI X, instead of SDI X & SDI Y for all available inputs.
In case of the DeckLink 8K Pro, it should look like this (SDI X In or Out):
