Snapshot Dependency with Branch doesn't work

In the documentation "What's new in Teamcity 7.1" you wrote: "If a build configuration with branches has snapshot dependencies on  other build configurations, all builds from the chain will be marked  with this branch when the first build is triggered.".

I have a configuration A, which i start by HTTP-Request with custom parameters (&name=vcs.branch^&value=branches/v1.2; parameter "vcs.branch" is used in the VCS root). This configuration has a snapshot dependency to 2 other configurations (B, C).

When i trigger configuration A it is built from the specified branch "branches/v1.2".
However configurations B and C are built from the trunk.

How do we have to configure B and C, that they are also built from "branches/v1.2"?