No Custom World Type

Working as intended, read the release notes https://minecraft.net/en-us/article/minecraft-snapshot-18w06a

"... don't work in this snapshot. Sorry. We have plans to re-add them in a much more extendible way, but unfortunately we were not able to have that ready yet. You will be unable to open any world of type "customized" in this snapshot, it will tell you to revert to an old version to play in those worlds."

is hardly equivalent to "working as intended." This is still a bug, still present in 1.13-pre4.