I suppose this is a bug.
I have WindowDuration set in the config, and I observe that m4fv files in the DVR storage folder are deleted as they go outside the window. My stream is video-only, so there are no m4fa files to speak of. But occasionally m4fm and m4fd files are created (I suppose it is when there is metadata in the stream), and they are never deleted. They remain in the "HHHH_MM_SS" folders along with the "dvrManifest_HHHH_MM_SS.txt" file.