Indeed, one need to help fight the Spawn to get the new Belt....

I got late to the New Spawn Party, and the Boss had just popped up (still at full health or so...), I helped taking the Boss down, so did a lot of damage and had looting rights yet, no new Belt earned...

So, I can definitely confirm that it DOES IS necessary to also help work the spawn in order to then be able to earn a Belt.... helping killing the Boss alone, does nothing....

Which is VERY good, thank you Developers !!

What I am unsure, is whether one needs to do "consistent" damage throughout the Spawn, or if killing a few baddies here and there for each wave will qualify for a belt drop...

Anyone knows whether the contribution to working the entire Spawn needs to be significative, or can it be just "symbolic" whereas killing a few baddies from the waves will work just as well ?

Ya I am glad that is a fact.

last night I got there & was killing the stuff from the 1st wave
and realized that the 2nd wave had already started.
I got the belt when the boss died.

the round before that, a couple people showed up on the 3rd wave 
and didn't get credit from the boss (even though they were killing all wave demons)
they had to wait until the next spawn to get the belt

I think there is a bug on the spawn (if someone can confirm) that if a spawn is worked from wave 1 to another wave (likely 3 or 4... not sure) but then the champ is not spawned and the candles all go back down to the first wave (ie whoever was working it leaves) then a new person working the spawn from that reset doesn't get any spawn drops (masks). 

I was on Origin earlier when ATL was down and was working the spawn from level 1 (nobody around) and got it up to level 3 without getting a single mask. In comparison to yesterday on ATL; I killed about 1/8 of the spawn I did on Origin and received 3 masks. Understand there can be RNG so that's why I wanted to see if anyone had experienced the same thing as it feels like more of the participation drops spawn for this champ spawn than previous ones.

If this is a big it could be problematic as people who might be able to work the spawn up to level 3-4 might leave if nobody comes to help as they know they can't push it to spawn the champ and/or they know they can't solo the champ and new comers might not help if they won't get anything if the spawn is at level 3-4.

I did two spawns while posting pics and making gates. I did not get the drop on those.

It depends how many other players are there also.

I got 9 belts out of 11 spawns yesterday so it works fine.

I gave up trying to get drops on 2 characters at the same time. I'm not coordinated enough to kill enough spawn for both.