Blackmagic answered my mail. They said this:
Speaking with our developers it appears we use the same method for processing that we do with our Teranex processors to process the image on the UpDownCross, which has been found appealing to most customers.
It creates a motion blur instead of a combing effect or field dominance kind of look that you are looking for. I have logged a feature request to see if this would be a selectable option, this will be discussed by the development engineers,however I cannot guarantee if and when this may be implemented.
Good that I just returned this conversor. It's no use for me like that.
I'd mainly use it to do live broadcasting for TV from the BMMSC4K to SD-SDI (625i50), among other things.
And TV channels usually want the interlaced footage to look like interlaced footage (you know, double temporal resolution) for sports or basically everything if they work that way. Even more reason if using a jib or crane due to the kind of movement, as it's my case...
Discarding half of the fields disregarding the output format is just seriously wrong in any case I can think of. Not everything is 24p, 25p or 29.98p...
I'm going to purchase a Decimator MD-HX or something else.
Too bad this conversor didn't work as advertised. It could've been a really good value.