upload-to-bookworm-backports does not find backported build dependencies

In https://debusine.debian.net/debian/developers/work-request/555601/ 7zip requires dh-exec >= 0.30~; bookworm has v0.27 and bookworm-backports has v0.30.

The build fails:

Install main build dependencies (apt-based resolver)
...
The following packages have unmet dependencies:
 sbuild-build-depends-main-dummy : Depends: dh-exec (>= 0.30~) but 0.27 is to be installed
E: Unable to correct problems, you have held broken packages.

Locally, I need sbuild --build-dep-resolver=aptitude for the build to succeed (as per sbuild wiki).

From #1237 it seems this should be supported already for the build part, so maybe this is an error specific to bookworm-backports.