That probably is a safety feature, USB-C delivers 15 watts of power = 5 volts at 3 Amps.
USB-C with PD Gives 100 watts = 20 volts at 5 amps.
If something goes wrong, either one of these is more than enough to send you ‘pining for the fjords’.
So if you shoot in high speed shooting, either 100p to 50p or 100p to 25p, and use a 1/100th shutter speed, the screen is almost black, like the shutter speed is 1/4000th. Crank up one step to 1/120th shutter speed and the image is exposed, and each step up slighty darkens 1/3rd of a stop like it should. 120p high speed is fine, but the 100p and 1/100th shutter speed doesn't work.