Weird. I can start the game up in offline mode just fine.
I just forced my connection offline while playing, too and nothing happened.
Must have just been a random glitch or something held over from the GFWL days,
Yeah it must be a bug. I can still play offline if I just start over from the main menu but for whatever reason it takes me back to the menu if it goes out while playing. Thankfully this game saves after entering every room.