qmake *= operator (Adding Unique Values) overrides previous value

trying to build qt with QT_DISABLE_DEPRECATED_BEFORE=0x050F00 as a start to migrate to qt6. (by the way, a question: should i build both qt and my project with this define or only my project? in any case the report is still valid) and i'm seeing strange errors. figured out that \qtbase\mkspecs\features\qt_module.prf is overriding my define by the following line:

DEFINES *= QT_DISABLE_DEPRECATED_BEFORE=0x040800

so i tried the following: added another value right after existing in that file. like that:

win32 {
    # On Windows, due to the way DLLs work, we need to export all functions,
    # including the inlines
    DEFINES *= QT_DISABLE_DEPRECATED_BEFORE=0x040800
    DEFINES *= QT_DISABLE_DEPRECATED_BEFORE=0x050800
} else {
    # On other platforms, Qt's own compilation goes needs to compile the Qt 5.0 API
    DEFINES *= QT_DISABLE_DEPRECATED_BEFORE=0x050000
}

and to see the result in some .cpp file i've also added this:

#define QuoteIdent(ident) #ident
#define QuoteMacro(macro) QuoteIdent(macro)
#ifdef QT_DISABLE_DEPRECATED_BEFORE
    static_assert(false, "QT_DISABLE_DEPRECATED_BEFORE defined " QuoteMacro(QT_DISABLE_DEPRECATED_BEFORE)"\n");
#else
    static_assert(false, "QT_DISABLE_DEPRECATED_BEFORE not defined\n");
#endif

and i see this as a result:

error C2338: static_assert failed: 'QT_DISABLE_DEPRECATED_BEFORE defined 0x050800'

which means that operator *= does override previous value.

tried this on 5.15.3 and 5.15.8.