Adventure mode Chapter “Two Worlds” generates both a normal and sick wormhole

AFAIK this isn't a bug. There's just a chance to get a normal wormhole, it's intended behavior. See the wiki article.

I think it's an oversight in the worldgen code, something that's working correctly but shouldn't be applied here, rather than either a bug in the sense of a piece of code that was written incorrectly or a fully intentional effect.

I don't think I've ever been on a Two Worlds map that didn't generate with a regular wormhole, which undermines the gimmick of the level considerably. (This goes back to before Shipwrecked, since I started playing between RoG and Shipwrecked.) I would prefer if it remained possible for the two islands to generate with either a small land bridge or a regular wormhole connecting them, but there was no more than a 10% to 25% chance of that happening so that it remains a rare lucky break, instead of always or almost always like what happens now.