The Exploit with Wonder Mail S Generated Jirachi's Challenge Letters: How does it work?

There's an old topic on here that mentions a strange exploit where, using the advanced mode of the wonder mail s generator, you could create many Jirachi challenge letters to unlock any dungeon in the game, allowing you to revisit one-time only story dungeons, demo dungeons, and even dummy/glitch dungeons.

I've tried to generate a letter that lets me unlock the Hidden Shopkeeper Village demo dungeon, but I strangely generated a dungeon called Guiding Sea that I can't even visit. Another attempt resulted in a game freeze attempting to load a dungeon called Dummy. The original post and comments themselves don't even mention how this exploit works or how to correctly generate the dungeons you want. I can't seem to get the Hidden Shopkeeper Village no matter what I generate.

Does anyone here actually know how this works and can provide any kind of information on this exploit glitch?

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