Hello! This behavior is due to python-apt-common not being marked as Multi-Arch: foreign. I have already filed a bug report at https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=968458
If you would like to continue, you can modify the python-apt-common entry in /var/lib/dpkg/status to include Multi-Arch: foreign and run crossgrader again. I'm not quite sure why this is stopping crossgrader on Bullseye but not on Buster; I'll look into it.
