Thanks — I traced the spawn path for that dungeon.

Every monster in Cave of Mushrooms (both mushroom types, 25 of them, and nothing else) is placed by the same routine: it downloads their sprites, then waits for the loader to report "finished" before putting a single one on the map. That wait had no time limit. If one sprite request hangs — and the art host is unreliable for old dungeon artwork — the wait never ends and the whole map's monsters are dropped without any error message. You just get an empty dungeon.

The sister routine that spawns boss summons already had a guard for exactly this, with a comment saying the same wait used to hang "for good"; the routine that spawns a map's normal monsters never got it. I've added the same guard, so a stalled sprite now costs a few seconds instead of every monster on the map.

If you do see it again after this ships, please open the browser console and grab anything starting with [Mob] — that will tell us which sprite is hanging.

I could not pin this down to one defect from the code alone, and I'd rather record what I found than guess at a fix. Two things can each produce exactly what you saw, and I can't tell them apart without a repro. (1) The Cave of Mushrooms is stocked entirely by Zombie Mushrooms, and that sprite is served only from the MSU asset CDN — it has no local copy bundled the way the trickier mobs do. If the art fails to arrive, the monsters are silently skipped and the cave is empty. (2) In a shared world, one player is the 'host' who simulates the monsters and everyone else converges onto what the host reports. That's by design and it's how dead monsters stop being ghosts on your screen — but it also means that if the host's copy of the cave is empty (because their art failed, or they'd already cleared it), everyone else's monsters get removed a few seconds after they walk in, which reads exactly as 'monsters vanish after entering'. What would settle it: does it happen when you're the only person in the game? If yes it's the art; if only when others are online it's the host sync. Either way I don't have a patch I'd trust.

Fixed. The cause was a lie on the wire, not a bug in your client: whoever is hosting the map's monsters broadcasts a snapshot that includes the AUTHORITATIVE monster count, and it was broadcasting that count while its own monsters were still loading (the mob art comes from the CDN, so for the first second or two on a map the host honestly has zero). Everyone else read "this map has no monsters", removed the monsters they had correctly spawned one by one, and the map stayed empty — respawning is the host's job too, so nothing ever came back. The snapshot now carries the host's own "I have finished spawning" flag, and the vanish-cleanup only acts on a snapshot from a host that says it is ready. The host keeps sending while it loads (so positions and HP still sync), and the ready flag trips automatically after 15 seconds even if an art download hangs, so a stalled texture can't freeze anything. Bonus: a player who walks into a map the host has already cleared now converges immediately instead of seeing leftover ghost monsters.