This has happened in another world as well.

 What I expect to happen: spawning on dry land.

What really happens: I spawn underwater with nothing anywhere near me (there is a nice jungle at 1402 42, though).

The seed for this world is -5366172275149697454. In the end, I had to switch open the world with LAN (with cheats), go into Creative, and fly for a long time before I found the weird microbiome in another post. The coordinates of spawning were 8 (something close to 63) 8. This happened in an experimental snapshot as well (the first one, I think).