TIL: In pg_dump, beware the combo of "-Fd" and "-Z"

PostgreSQL 12.1-2.module+el8.1.1+4794+c82b6e09 from the RHEL 8.3
repository.  (Installing software outside of the RHEL is Against Policy.)

For a project using 9.6, I have this command backing up a database:
pg_dump -d ${DB} -j ${THREADS} -Fd -Z0 -v -f $DB 2> ${DB}_pgdump.log

So, for the 12 project, I copied it over, removing the "0", since I want
these backups compressed.
pg_dump -d ${DB} -j ${THREADS} -Fd -Z -v -f $DB 2> ${DB}_pgdump.log

Since the -Z default is "6", I assumed that would compress the backup.  It
didn't, acting instead like "-Z0".  Removing "-Z" made it compress.

This surprised me, and is very counter-intuitive.  A bug?

--
Angular momentum makes the world go 'round.