For further context for anyone else looking into this, this seems to be caused by the mapwithai JOSM plugin rather than JOSM itself. The source tags were added at upload so I never got a chance to see them. It's also been very intermittent, only happening to some changesets. I've been having several issues with my current workflow involving mapwithai for importing LINZ buildings, when they affect the uploaded data that isn't acceptable so I've stopped importing for now until I can find a better workflow.