ok thank you for looking into itThat's actually a bug. My Treasure Bag drop code checks if an NPC is a boss and changes the loot accordingly, but it looks like Terraria doesn't technically consider Betsy a boss. So it drops it's usual loot.
I can easily fix that, though.
The 6th slot should work, that's weird.
I tested this a while ago and it looks like it's an error with the Mod Settings Configurator mod. Without it I can run a server with VanillaTweaks just fine.
I reported the error to him but he hasn't fixed it so far.