Did you link to the correct page on VGA legacy?
I got this:
http://www.vgamuseum.info/index.php/component … nologies-f65550
All those cards appear to have some kind of Video in/out capability.
That golden RCA connector and probably some proprietary breakout cable for the middle connector.
Should be interesting what happens if you attach a TV to that RCA connector - many TVs should still offer that input, maybe via a SCART adapter.
Video I/O was rather early or exotic, that being from 1996. Maybe they screwed that up somehow.
That small AD chip on yours in the upper left corner seems too primitive to provide a "video out" all on its own.
OK, that's what it does:
https://www.mouser.de/datasheet/2/609/AD724-1501848.pdf
But part of the work will be done by the main chip, other than VGA.
Maybe that's prompting it to provide some exotic VGA modes or messes with DDC communication.
At 1996, it's probably still older than any of you TFTs?
I don't think it's a malfunktion, but if so, there might be one that fails to get the monitor started, not providing a certain signal.
That would fit the fact that the capture device still gets a picture.
Someone here will know much more about such things.
What I would try:
Install that thing in a more modern windows system along the main card, use it as a secondary VGA and see if you'll get a picture at certain settings. If you manage to find a driver...