I've taken several screenshots in an imgur album to help explain this. https://imgur.com/a/tewX8JY (most relevant screenshit included directly in this post) I hope that helps, because I'm not the best at explaining.

tl;dr: Nether-ravines (and possibly other world-gen) cutting into lava-oceans and creating floating lava source-blocks everywhere.
Repeatable (for me) by starting a normal world, going into a Nether-Portal in creative or survival, and looking around the edges of the nearest lava ocean.

Have-Time;Will-Read: I was playing through a survival game on the snapshots to get a feel for the new mechanics. I finally progressed to the point where you can go the Nether. I made the portal just about surface-level, and walked through. The Nether took it's sweet time to load (as it pretty much always has) and I started exploring.

First thing I noticed was a large amount of ravine-like world generation similar to the surface. I gotta say, the nether looks pretty cool with all those ravines. But whenever they started bumping into the lava ocean, lava-source-blocks from said ocean would be floating in the air. At first I thought it was just another basic glitch. So I went on with my game.

But then I found a whole area I could walk around in that was completely covered by where the lava ocean would have been, and realized it wasn't an average glitch. I loaded up a new normal world in creative, and made a portal to the Nether straight-away. Same thing there. I tried this about 6 times with the same results, then decided I might as well see if this is a known bug.

I followed the bug-reports link on the Minecraft.net snapshot description-page and searched for it. After half an hour of trying (but not succeeding) to find a similar bug, I took several screenshots, signed up for the place, and wrote up this report. Sorry it ended up being so long-winded; but I hope it helps.