[2020] Hunter Mask didn't drop for me, but did for my team member

Running the Cross, Diamond, Death and Phantom hunters last night. We killed all 4 hunters but Diamond was the last one to go down. my buddy was down, and I was finishing off diamond when a control point NPC wandered in an also was attacking. I am not sure if she got the last shot in, but he went down, and didn't drop his mask. I got the other 3, but not diamond. Also no keys dropped for any of these hunters for us. my buddy only has 2, and I only have 5, and we should have 10 masks done. I have 9, he has 10. has anyone else seen a hunter not drop a mask? is the only way to replay this to find someone who has not done it yet?