Still no luck but wanted to add that I tried changing the Post Process Material from Opaque to Transparent. This allows me to, just to confirm, connect the Alpha pin of the last Over node to the Emissive pin to confirm that the alpha pin has the correct alpha signal. But when its connected to the Opacity output pin it does not affect the key output.
Also I was able to test this also with a Decklink 8k Pro and it behaves identically to the Duo 2.