jsonlog logging only some messages?

I tried loading the jsonlog module from
https://github.com/michaelpq/pg_plugins into Postgres 9.6.

However it seems it resulted in logs only for session log messages but
not any background worker log messages. We have log_checkpoints set
but there were no log messages in the json log about checkpoints. Nor
were there any from autovacuum.

Also I have log_destination set to stderr,cvslog and as I understand
it the jsonlog module effectively overrides the "stderr" output which
goes to the file named *.log (which I find super confusing,
incidentally). But I was expecting to get the csv file as well. We
didn't, we only got the *.log file with the (partial) json logs. Is
there something I'm missing here or is this unexpected?

--
greg