Running with -XX:+CMSScavengeBeforeRemark will do an extra call to GenCollectedHeap::do_collection() inside CMSCollector::checkpointRootsFinal().

Inside CMSCollector::checkpointRootsFinal() we already have a valid GC id set up, but the call to GenCollectedHeap::do_collection() will create a new one using a GCIdMark, which means that when it returns the GC id is set to undefined.

It is correct that GenCollectedHeap::do_collection() creates a new GC id since we are doing one extra GC there. But it should not reset the GC id to undefined when it is done.