Comment 4 for bug 1377142

There does not seem to be clear documentation on this. Just a note that from your description I was not completely sure whether you did unmount before starting the release procedure. I would say unmount is a must.
Doing so, I also ended up in the state where bcache0 still existed. But I was able to get rid of that by

echo 1 >/sys/block/bcache0/bcache/stop