It's disabled when the configuration format is different between them, as marked by the "<version>XX.Y</version>" in config.xml
If the configuration version is different, they cannot sync because it could push incorrect data.
That said, synchronizing between different versions has never been officially supported, nor recommended. It may have worked by chance before, but we never recommend running different versions for any measurable amount of time. Just long enough to make sure the updated node is functional/tested, which shouldn't involve any configuration changes.