One possibility has to do with a patch. There's been issues with death root. I read that it was supposedly taken off of the nursery seeds list because you couldn't actually harvest seeds from the plant. That was a glitch that I guess they addressed recently, but use like the other glitch regarding the arch demon mosaic, the patch didn't actually fix all problems it claimed it would.
That's my best guess
I'm glad you mentioned the Archdemon mosaic, because that was exactly the other issue I was coming to find information on. I completed all the mosaic collections, but I'm not getting credit for that one -- the mural's missing a piece and the codex won't update. Bioware, I love you, Y U NO FIX? 