I'm not sure if it's a bug or a feature I just don't quite understand but when i used the Loot Box to do those quest and came back the first time from the dungeon it was gone from my house. I can still do the quest after it disappeared but I'm not sure if I'm getting anything and its just a random item in my inventory that I haven't noticed or I saw in the comments where furniture sometimes disappear in the house and that's what happed and do need the chest to collect the loot boxes I earn?
Yes, I've been informed of the furniture disappearing, unfortunately the script I'm using appears to happen on certain computers/devices and there's currently no fix around this.
You will need a chest to collect your loot from the loot box itself.
Fortunately, I added the Loot Box to be purchasable from the furniture store at a low cost and other essential furnitures in case anything like this happens in the future. Hope this helps a little in resolving your disappearing furniture.