Let's make sure we agree on what the root problem is. There's a software development ecosystem built on parsing the FTL JUnit XML reports. I can't have the format changing silently in production.
I'd expect some type of heads up that the XML will be changing. I know bitrise would also be interested as they're building a dashboard based on parsing the XML.
If FTL is going to continue to make silent changes, then setting fail on unknown would be a reasonable work around. At the moment, I consider silent changes a legitimate bug that impacts multiple downstream businesses and tools.