Heya,
Yeah it's sort of expected behavior. Technically even a known bug.
HLS had to work slightly differently at the moment in order to make sure that all sub-playlists were being ingested. There are plans to make this work better, but at the moment MistServer needs to ingest the full playlist and doesn't really stop unless specifically told to.
The next version of MistServer should have this patched, but at the moment you would have to manually stop the ingest.
I believe the stop sessions "might" do this, but chances are that you need to use MistUtilNuke -s streamname in order to stop it fully in the current release.
With kind regards,
Balder Viëtor
Head of Testing
MistServer