I am putting this in tribble forum in hope that it can sneak it's way into S7 update.
My suggestion is simple:
Please change PvE que for STFs so that you do not get loaded into an existing team that has had someone leave. While it doesn't happen that often, it really isn't fair to be dropped into a group that has already made a mess of things.
Just give the leaver penalty to the first person to ditch, and let the ones left behind leave and try again with no wait.