autovacuum worker started without a worker entry

Hi all,
I occasionally see the message "WARNING:  autovacuum worker started
without a worker entry" in the logs.
From what I can see here
<https://github.com/postgres/postgres/blob/master/src/backend/postmaster/autovacuum.c#L1678>,
the launcher forked a worker and in the meantime the launcher decided
the worker is no more useful. If that is right, I'm guessing why the
worker should not be useful anymore: since a single worker could work
on a table, the only reason I can find is that someone run manually
vacuum within that database/table.

Moreover, I've a question about emergency autovacuum (wraparound).
Since autovacuum workers are started by postmaster on a signal
received by the autovacuum launcher, and since with autovacuum = off
the latter is not running, how does the postmaster decide to start and
emergency autovacuum?
I only found what seems to me a normal autovacuum start at
<https://github.com/postgres/postgres/blob/master/src/backend/postmaster/postmaster.c#L5263>.

Thanks,
Luca