Had the same issue when i first got the Decklink.
I even downconverted the HD material in AfterEffects and recorded the file on DigiBeta and also recorded straight from HD Downconversion to DigiBeta.
I then re-captured to uncompressed….put the two files on the timeline and was amazed at how much the realtime downconversion does flicker removal and thus loose too much details.
I also checked with BMD support an been told that i cannot compare realtime downconversion with a software that has all the time to render.
I guess it’s done by hardware and cannot be fixed by a software driver update.