LGrane, i see that that opto-isolator inside your box outputs 12V 1300ma. This is 15.6W and sometimes it may be not enough for Pocket 4K camera which consumes up to 22W at maximum resolution/framerate/brightness. Hope it will work for you.
HDMI cable shields are very different. I checked a lot of different cable models and cheapest no-name cables always have voltage leak between grounds. Same timev oltage leak on quality cables always was very close to none.
Take attention to this article
https://www.newsshooter.com/2020/02/15/ ... i-outputs/ and this forum thread
viewtopic.php?f=2&t=107642 It seems we have now a real explanation what exact cause HDMI damage when camera and monitor are powered from same battery:
From the ARRI article:
"This damage to the SDI connection can occur when connecting an unshielded power cable to an accessory that is already connected to the SDI output of the camera. If the plus pin is contacted before the negative pin, the BNC cable will close the current circuit resulting in the fatal damage of the included SDI driver chip which itself is not designed to handle such high currents. "
If simplify worlds - to avoid damages, you should always connect (-) DC power cable pin before (+) DC power cable pin.
Actually it is not so easy to do in real life until you have dedicated switch on positive wire of DC cable, or specially designed DC connector which 100% always connects (-) pin before (+) pin.