I have the same problem. To me it seems like it started when i upgraded the software and firmware, not shure wich version though.
To me it works with two signals but as soon as I connect a third one the multiview is gone. I use 4 JVC GY HD251 over 720/50p and a macbook pro for graphics.
The program monitor works fine. Ive tried to switch cables between the program monitor and the multiview and the program monitor always shows the picture.
Its really frustrating not having a clue what the reason for this is. Does anyone have any kind of solution? I have emailed BMD but haven’t got any answer yet.