I found it also very easy to confuse Ancient Orc and Ancestral Orc when opening the boxes. So that may skew the perception of Ancestral motifs dropping as well.
Another thing I've noticed that correlates to this list is that the style mats for the respective motifs match whether the motifs drop or not. We don't get ones like Amber Plasm (Soul-Shriven), Lustrous Sphalerite (Militant Ordinator), Ancient Sandstone (Ra Gada), and such, because their respective motifs don't drop either. And the style stones are much more common than the motifs themselves (see also: Desecrated Grave Soil vs Worm Cult motifs)
so with a sample size of (so far) approximately 2500, I have still not seen the following trait stones appear from a box:
Carmine Shieldsilk (Shield of Senchal)
Aeonstone Shard (New Moon Priest)
Etched Corundum (Ancestral Nord)
Etched Manganese (Ancestral Orc)
Etched Adamantite (Ancestral High Elf)
Etched Bronze (Ancestral Reach)
Arkthzand Sprocket (Arkthzand Armory)
as well as the usual suspects (MO, SS, Ra Gada, Ebony, the holiday motifs, the crown motifs, the basic book motifs).
But maybe it's just bad luck; if someone sends a screenshot of one of the above motifs coming from a box I'll happily update it.