[Ryu-Kazuki]

There's nothing I can really think of that causes it, since the battle generates like normal, except without the monsters in it. Unless it has something slipped up in the generation process? Because this doesn't happen with just monster patrols as the OP states. It's happened in both Core battles and Jammer Cavern battles too (not sure about boss battles, though, and let's hope it doesn't). That's the only thing I could think causes it to happen.