In my test, I'm rsyncinc data from my /etc and /usr directories on to my test dataset.
I can always reproduce this (with latest spl/zfs) by creating 100k files (with a total of 100 GB) in a zpool root dir, and deleting them totally afterwards.
No need for xattribs, and no cleanup with export/import.
I've discovered that some objects remain in the root dataset after deleting all the files in it.
In my test, I created a pool name junk, copied a bunch of files into it and then removed them.
I guess the output of zdb -d would still be interesting and also it would be interesting to know if an export/import cycle makes any difference.