I’m not sure if anyone here has addressed this yet. But when playing Spartan Ops solo on legendary on the MCC, I noticed that when I quit a chapter part way through to change my load outs, the game said that I had completed the chapter. I started and quit chapters for a whole episode and found that it still gives you the achievement for completing the episode even if you didn’t finish the chapters. I’m not sure if this happens on co-op or on any other difficulty, but decided that it was a bug that could easily be mistreated. Also not sure if this is the right place to post and realise that some people will probably see this and go to complete all the episodes. So fix it before it’s too late!
Thanks for the heads up! I’ll hurry up and do them all before the patch. Who wants to sit there and do 50 legendary missions solo? Great find! I’m sure a lot of people will be thankful for you bringing it to our attention! You da real MVP! 
This also applies to the custom games to get the multiplayer wins and related achievements.