Something in Eternal Grove I don't understand?

Well, just today I was doing Eternal Grove with h/h for the first time (in normal mode), going for masters. I read up on it on wiki, made some builds I thought were good, and started. Now, through most of the mission, I was doing pretty well, managing to stop all the invading forces before the tree singers died. I made it to the last group of afflicted, and all the tree singers were still at full health, and i pulled all of the heroes and henchies out to face the last group. That was going quite well too, with all the afflicted falling pretty fast. Finally, only the sin boss was left, and, as I said before, we were outside of the hill. Suddenly, the north tree's health began to fall fast, and suddenly i got the message that there were only 11 tree singers left. I was with the sin boss on the outside, with only the center tree even remotely near us. I thought that perhaps I'd let an afflicted through, but when i killed the boss a couple seconds later, the cut scene triggered and the mission ended, meaning that there were no other afflicted left. I was just wondering if anyone knew why this had happened; I'd rather not go through all that stress again just to have masters robbed from me at literally the last second again. Does the sin boss have some skills that can damage over distance? Any help would be appreciated.

P.S.-I didn't post this in the bugs forum because I don't have any reason yet to know that it wasn't just my own stupidity, so please don't ask "why isn't this in the bug forum?"

some kinda of new degen?

but this sounds like a glitch

The Afflicted come in 12 groups. You have to keep all 12 Singer alive while thsoe 12 groups attack. All 12 groups do not come at one time. The last 2 groups, are what you had trouble with. Group 12 has the Sin and Rit bosses. Group 11 has 3 Warriors. There is a slight delay for when the 11th group shows up, and you probably saw the 12th group while they were still coming. When I do that mission, I always leave 1-2 people at the west gate to guard against those warriors.

Although you may not have killed those warriors in the 11th group, killing the bosses in the 12 group completes the mission.

Yeah theres two warriors that come from the left side which is probably what happened.

After trying to get master on this on my own I decided it would just be easier to do it with a guild mate, was a breeze, I would recommend just trying to get someone to help you out with their heroes, guaranteed master.