The game is set up so lava pools spawn before ruined portals in the Overworld. This means you get some weird stuff going on when a ruined portal spawns underneath a lava pool.

My phone won't let me take a Screencast. Here's the world and location to see for yourself.
Seed: -1260790447
Go to:
X = -445
Y = 55
Z = 22
And you will see what I mean.
Basically whats happening is the lava pool generates, and then somewhere down the line to portal generates, and because the portal spawns later than the lava, part of the wall is lava sources, but the lava hasn't been updated, and so you get this weird phenomenon.