Check if this is true story (probably is).

Package: release-notes
Severity: normal

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 like

deb http://security.debian.org/debian-security bullseye-security main

(adding /debian-security was proposed in [1]).

See [2][3][4] for some more information.

I should probably also sent a mail to d-d-a@ or so about this in the
near future...

Ansgar

[1] https://lists.debian.org/debian-devel/2015/12/msg00333.html
[2] https://bugs.debian.org/614204
[3] https://lists.debian.org/debian-devel/2015/12/msg00254.html
[4] https://lists.debian.org/debian-security/2019/06/msg00015.html