I got my 12th and final mask last night having been missing the cross one for a while.
After playing heroic tidal I just stayed on the leaders world, went around collecting stuff, killing stuff, eventually night fell and luckily the other 3 had logged off so I went to the Christmas tree and was able to pull the lever. Only 3 spawned that I could see, first one killed just dropped gear but the second one was the cross hunter.
12 masks and 8 keys now all got.
Nope opend case yet. Best to wait until next patch ????