Hi,
I have a DeckLink 8k Pro fully functional, double channel fill+key
for me the trick was in the blackmagic settings: 1ºin+2ºout and 3ºin+4ºout
in this way the f+k are linked together and perfectly in sync
(external keyer)
I tried only in 1080@50i, I don’t have any way to test higher resolution at the moment