I built qbs and then rebuilt it with some completely irrelevant project property set to a different value, as to force re-resolving the project, upon which lots of files were re-compiled for no apparent reason. Looking closer at one of the artifacts, it became apparent that its command line changed from this:

...,  "-DQT_SCRIPT_LIB", "-DQT_CORE_LIB", "-DQT_NAMESPACE=QtNamespace",  ...

to this:

..., "-DQT_CORE_LIB", "-DQT_NAMESPACE=QtNamespace", "-DQT_SCRIPT_LIB", ...

So here the order of cpp.defines appears to be somewhat random.
Not sure why the problem does not manifest itself in the autotests.