Only one historical exception: Cygwin, basically inertia from the Cygnus acquisition.
Maybe more significantly, Red Hat has only made limited use of CLAs in the past and hasn't used any CLAs for many years now. It's basically corporate policy.
There was one other Cygnus-era one... libgcj, the gcj runtime library. However, Red Hat assigned copyright to the FSF in exchange for them adopting a more permissive license for the GNU Classpath project; one that was eventually selected by Sun when they open-sourced Java.
Maybe more significantly, Red Hat has only made limited use of CLAs in the past and hasn't used any CLAs for many years now. It's basically corporate policy.