I have the camera connect to the Blackmagic UltraStudio Recorder 3G and then connected to the Mac Mini with M1 chip by way of HDMI.
So, the BM box should be connected to your computer by Thunderbolt, not by HDMI.
Since the UltraStudio Recorder Mini is equipped with Thunderbolt interface, this can quite easily be mistaken for a USB-C connector for camera connection. The correct way of hooking this up would be
Camera HDMI output -----> Mini HDMI to HDMI cable ------> BM UltraStudio Recorder ------> Thundebolt 3 cable ------> Mac Mini
Still, I don't believe this is where the fault is, I recon this is just a typing error in your description of how you've hooked things up.
From my recollection, the default HDMI output of the Canon camera is 720i. If you've installed the Blackmagic Media Express software, you should be able to open that up and see what resolution the UltraStudio Recorder is picking up (it is auto sensing the output from the camera). If it's 720i, you should change that to whatever output you'd like it to be -
in the camera.
Here's how it is described in the G50 manual: