libseccomp in buster-backports is currently at a version from halfway
through bullseye development. Please could you update it to the version
released with bullseye?
It's looking as though this is going to be a prerequisite for updating the
version of flatpak in buster-backports to catch up with bullseye-security
when the recent fixes for CVE-2021-41133 arrive in bullseye-security.
A trivial backport of the version in bullseye seems to build successfully
and works for at least Flatpak, but I am not a libseccomp expert and
I don't know what additional testing is considered appropriate for
that package.