QbsProjectManager: adding properties to Qbs build step works not as expected

Steps to reproduce:

• create a simple Qbs project
• use a kit configured to use Qt4 (or adjust the example below)
• put a property (bool) inside the main qbs file and set it to true
• add a condition to the current product which uses the defined property (I've got something like this: )

condition: {
            if (project.myVar) {
                if (qtCore.versionMajor > 4)
                    return true;
                else {
                    throw("This project needs Qt5");
                    return false;
                }
            } else {
                print("Limited version will be build (Qt5 features missing).")
                return true;          
           }   

• trigger a parse/build of the project
• switch to 'Projects' view, open build settings, Qbs build step
• for properties enter 'project.myVar:false'
• try to rebuild the project (should now no more build)

Current result:

• still uses the old settings (cached?)
• Qbs clean on the project (triggered via context menu) does not help
• Reparse Qbs on the project (triggered via context menu) does not help

Expected result:

• at least after a Qbs clean or reparse this should be possible

Note: If I'm manually removing the build directory the expected result comes up.