LK-99 is a quasi two-dimensional material, and one which seems particularly challenging to synthesize in a bulk polycrystalline three-dimensional mass. Even the original researchers stated that they'd "get it right" only a fraction of the time, and that their famous floating cracked-disc is still impure. I think that there's no reason to assume that negative results right now are conclusive, whereas positive results…
Hey, what does this actually mean? Like, space is 3D and I don't think you're claiming all LK99 samples are all literally-one-atom-thick, so clearly it's a 3D object.
I'd guess it means that the material has very regular layers, but (much like graphene) I've never actually seen someone directly say what "two-dimensional material" means 3-dimensionally.