Dual Display image shrinking

We have a Mac Mini being used in a dual display VC system and digital signage being fed to both displays. Right display is connected to the HDMI port on the Mac Mini, and the Left display is connected via an Apple USB C to HDMI adaptor. Image fills both displays as required  during VC mode , but when digital signage is engaged, the display connected to the USB C port on the Mac Mini shrinks the image, leaving a black border on all four sides. the only way to rectify the issue is to reboot the Mac Mini

Any insight would be greatly appreciated