
Here's what the inside of my PK3 looks like:

And here's the contents of the MAP01 WAD:

My MAPINFO code is really short so I won't post it here, but it does have declarations for custom map name and music.
I have no idea why it isn't working now. I had another test PK3 I made a few months ago that has pretty much the exact same structure, but it works. When I try to replicate it now I get this. My only guess is the original Doom Builder filename was "bigroom-cyberdemon.wad" and I renamed it from within Slade, but I honestly am shooting in the dark here.