The problem appears to be this: apt-listchanges and dpkg-preconfigure both try to prevent their own failures (and those of Perl) from causing apt to abort by sanitizing the exit code in apt.conf. However, it appears that despite a successful return code, apt is aborting the upgrade. As the bug title suggests, I think this may be due to problems writing on the pipeline in SendV2Pkgs, or something along those lines.
-- - mdz
