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?
The other thing to look at would be:
dense_ad = ad.cut_region(["obj['Density'] < 0"])
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.