Got Season 7 Update, wanted to play a social game after playing a bit of customs but several times the queue filled up but then and then it prompts me that I am leaving matchmaking when I never clicked to do so.
343 are investigating this issue:
https://twitter.com/HaloSupport/status/1407863257667227649
> We are currently investigating an issue in Halo: The Master Chief Collection that is causing players to get stuck during matchmaking then forced into “Leaving matchmaking…”
Feel free to use this topic in the mean time - also a good idea to follow Halo Support on twitter for further updates
https://www.halowaypoint.com/en-us/forums/85d779d52cfd46918b4f8b638f2e3c7b/topics/known-issue-season-7-update-matchmaking-errors/7e6c2cc6-6e80-4efd-9d24-8ec550019a27/posts?page=1#post17