(Copied from the post in GD):
This is a word of warning for those wanting to get transmogs or just like hunting for the holiday gifts: Do not under any circumstances let them get offscreen or teleport to town before picking them up or else they will despawn.
I had the unfortunate circumstance where I capped my blood shards in an ancient vault that had a ton of holiday gifts, and teleported to town to gamble rings. Big mistake. Upon returning all of the holiday gifts had despawned. A second vault confirmed that even letting them get sufficiently offscreen will despawn them.
If you want to get those gifts, pick them up right away or you’ll lose them.