Both of these occurrences have happened quite regularly since Survival merged with Dark Gaming.
The Brain of Cthulhu (pre-Plantera) then Plantera (post-Plantera) occasionally appear at spawn randomly. It’s a very peculiar bug I don’t understand, and hopefully it gets addressed somehow.
Meanwhile, sometimes Treasure Bags fail to drop from a defeated Boss, sometimes Treasure Bags disappear, and sometimes some players receive two or more Treasure Bags. Quinci and others assert this is caused by lag. Personally, I’m unsure how it is caused; I think there are multiple causes.