I haven't verified any of this, but I strongly suspect that you will only get this callback for objects in the generation being collected and are found to be referenced. An object in Gen1 won't be reported for a Gen0 collection, while an object that gets
collected in Gen0 would not be reported at all.
Objects will only be reported during collection and not when allocated, so a short lived process might not report any objects if in does not live long enough to trigger a collection.