Hi,
This is indeed odd! It's possible that the cut region is causing some oddness. Can you try doing this, and telling us how it looks?
print(dense_ad["ones"].size)
The other thing to look at would be:
dense_ad = ad.cut_region(["obj['Density'] < 0"])
print(dense_ad["ones"].size)
Since that's the complement, we'd expect it to return something.
It's also possible, but I would defer to Britton Smith on this (since he did the cut region work), that the comparison being executed is ignoring units, and potentially silently failing.