When you assign a value to the xcode.signingIdentity property nothing happens. What would be expected is that the bundle module calls the codesign utility to sign the bundle.
This is especially critical for iOS where applications that are deployed on an actual device need to be signed even during development.
Looking at the rule in BundleModule.qbs it becomes clear that codesign is never executed since it operates on output artifacts tagged 'bundle' which don't exist.
I attached a patch with my changes to the bundle and xcode modules that made it work again.