Ok, looking closer at this, the exact behavior is this: if you anchor the right/bottom edge of an object, its right/bottom edge will maintain a constant distance in pixels from the edge of the layout. Thinking about it, this is consistent with how the top and left edge anchoring works, and therefore expected behavior. I'm not entirely sure what the use case is, but that's probably personal myopia.
I don't think that my original expectation (that right/bottom edge anchoring maintain the object's proportional dimensions) was unreasonable. However, now that I reflect on it, I probably would have done Dumb Things with such a feature, so perhaps it's for the best.
Regardless, I don't particularly consider this a bug anymore. It might be the opportunity for a new feature (percentage based width/height toggles on the anchor behavior?), but definitely not a bug.