All the newly built stable images (announcement pending) seem to have bullseye-security listed in the sources.list. So, there is no problem there.
It is also best if users use bullseye everywhere in the sources.list instead of stable to allow FreedomBox to automatically upgrade to next stable release bookworm in 2 years in controlled manner. A typical sources.list file should look like:
deb http://deb.debian.org/debian bullseye main
deb-src http://deb.debian.org/debian bullseye main
deb http://deb.debian.org/debian bullseye-updates main
deb-src http://deb.debian.org/debian bullseye-updates main
deb http://security.debian.org/debian-security/ bullseye-security main
deb-src http://security.debian.org/debian-security/ bullseye-security main
deb http://deb.debian.org/debian bullseye-backports main
deb-src http://deb.debian.org/debian bullseye-backports main