Eli Zaretskii <eliz@gnu.org> writes:

>> From: Eric Abrahamsen <eric@ericabrahamsen.net>
>> Date: Sun, 17 Mar 2019 19:59:41 -0700
>> 
>> >     So it's a bug.
>> 
>> The value of `mail-header-separator' gets added to
>> `message-font-lock-keywords', which does the actual fontification. Using
>> setq to change the value of `mail-header-separator' did not change the
>> font lock keyword, despite setting the variable before message.el is
>> loaded (or is it pre-loaded?). Using the customize interface to set
>> `mail-header-separator' *does* work as expected.
>
> Right.  So I don't think there's a bug here.

Actually, I think message-font-lock-keywords can do better: