WebPresenter can't handle footage that VideoAssist4K can

The Video Assist 4K seems to have a magical ability to accept footage that other devices can't handle.

I am using the Panasonic GH5S. This is a fairly new camera (Jan 2018) and fairly high-end, but I am only using it at 1080p25. In an effort to get its HDMI output into my computer for livestreaming, I have tried:

• Magewell HDMI (USB dongle)
• Magewell Pro HDMI (PCI card)
• BlackMagic Web Presenter

None of these devices is able to receive the GH5S's footage - unless the footage is fed to it via the Video Assist 4K, in which case it works every time. I have tried this with many different settings on the camera and the result is always the same: its 1080p25 footage just appears as a blank screen unless it first goes through the Video Assist 4K.

I have also experimented with a different camera, the Sony RX100 (also running at 1080p25), and in this case the Video Assist 4K is not needed at all. All of these devices handle its footage "direct".

Does anyone have any ideas as to what is going on here?

GH5S might have HDMI EDID issues! Why capture hardware does not understand it.
Have you tried other output resolutions instead of 25P?