With QMetaObject::invokeMethod call with QGenericArgument. following error is displayeed. 
QMetaObject::invokeMethod: No such method TestClass::testMethod(QVariantMap)
Candidates are: .....

Same code when build with Qt6.5.8 works without issue.

Attached tiny example which gives following output with Qt6.8.2

QMetaObject::invokeMethod: No such method TestClass::testMethod(QVariantMap)
Candidates are:
    testMethod(QMap<QString,QVariant>)
Invoke with QVariantMap successful? false
testMethod called with map: QMap(("key1", QVariant(int, 123))("key2", QVariant(QString, "hello")))
Invoke with QMap<QString,QVariant> successful? true

With Qt6.5.8 it gives following

testMethod called with map: QMap(("key1", QVariant(int, 123))("key2", QVariant(QString, "hello")))
Invoke with QVariantMap successful? true
testMethod called with map: QMap(("key1", QVariant(int, 123))("key2", QVariant(QString, "hello")))
Invoke with QMap<QString,QVariant> successful? true