In qbs' own project file, we have this construct:

    Group {
        condition: project.enableUnitTests
        name: "tests"
        cpp.defines: outer.filter(function(def) { return def !== "QT_NO_CAST_FROM_ASCII"; })
        // ...
    }

Since 16404c356b, the value of cpp.defines in this group (and only this group) contains only the values directly set in the product; the ones coming from e.g. the Qt core module are lost. The effect in this particular example is that since QT_NAMESPACE is no longer defined, linking fails later.