Cave Worldgen: auto-generated extra room

[skittles sour]

I've been trying to do a minimalist cave map with just three main rooms: an exit room, a labyrinth, and an atrium (the bottom room is mis-labeled, it's supposed to be the atrium entrance room). But worldgen keeps on throwing in an extra room, the one to the top right. I thought it was because I must include a spawnpoint_master and a multiplayer_portal, but after including those the worldgen still spawned one in. Does anyone know why it's doing this?

[skittles sour]

I've tried adding buffer rooms, but the same mysterious mechanism keeps on intruding. I'd appreciate infinitely any ideas to solve this. Here's a detailed map of the locks and keys I'm using.