When fixing a bug that didn't draw walls when partially on cavern area (that dark mask on picture) a new bug appeared. As you can see corridors (green mask) don't really behave that well when trying to find a way in a round room. The reason is diagonal "corner" mask (white) which corridor routine avoids when it tries to find a straight wall (colored ones). This bug could be fixed with pathfinding I guess.
It's not unusual that fixing something creates new bugs, because of the complex relationship between different parts of the dungeon generator.