So in my experience you apparently need to add the primaryassetlabel files into the Project Settings -> Asset Manager -> Primary Asset Types to Scan or it basically ignores them completely.
It also seems to ignore the primaryassetlabel’s rules settings, so I ended up duplicating the Chunk ID, priority, etc, into that section as well.
Now that I’ve done this, the assets do seem to be going into their chunks as expected, but additionally they’re being duplicated into other chunks if there is a referenced asset that needs it elsewhere in another chunk! 
I wouldn’t say it’s “broken” per-se, but it is definitely poorly documented and perhaps unfinished…