So, fun fact: If your co-op partner [I was hosting] finishes up their looting fast and then immediately joins a league match before you open your chest, it apparently not only doesn't give you that loot for the chain completion, it also boots you from the chain- counting it as incomplete for you. Which, I guess, is good in a sense, you could redo it for the end reward you missed. On the flip side, the end reward is /kinda the whole point/ of running it in the first place :X
I was kinda irked :'( I've been seeing a lot of complaints from new players about it as well- apparently it happens any time someone leaves the party, but leagues don't even give a 'do you want to leave party' prompt, meaning they're especially troublesome.
