Cannot open or find existing worlds

For some reason, 18w16a doesn't see many of my old worlds. They show up and play fine in 18w15a (and every prior version). I've tried renaming the folders to match the world name ("world" or "rdp industries", depending on which of the many world copies/versions that I have in my saves folder).

Did something with world detection change in 18W16a that I am missing? I didn't see anything in the release notes to make me think so, hence why I'm reporting this as a bug.

I'll reply with screenshots later.