Check if this is true story (probably is).
For bullseye, the security suite is now named bullseye-security
instead of buster/updates and users should adapt their sources.list
accordingly when upgrading.
People should probably use something likedeb http://security.debian.org/debian-security bullseye-security main
(adding /debian-security was proposed in ).
See  for some more information.
I should probably also sent a mail to d-d-a@ or so about this in the
Ansgar https://lists.debian.org/debian-devel/2015/12/msg00333.html  https://bugs.debian.org/614204  https://lists.debian.org/debian-devel/2015/12/msg00254.html  https://lists.debian.org/debian-security/2019/06/msg00015.html