The inability to take screenshots in cutscenes might actually be a design choice rather than a coding error.
Though as to why, I don't know.
I sincerely doubt it. Since it worked in the beginning, then it was predictably borked so that you could circumvent it, and now it basically doesn't work at all. I think they just didn't bother fixing it, or decided switching it off completely would somehow make people forget it ever existed..
It would be in Biowares best interests to fix in-cutscene screenshotting for good..