Since I have a personal GFB world with some already set land to test this, I went and figured it out. You can click on each image to look at them more closely.
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Here are the pictures of my testing environment. This section of land is completely pure. The trees are still purple because I was not done scraping the purple paint off, they do not contribute to a corruption biome nor spread the corruption. A torch is held out with biome torch swap active, it does not detect any valid biomes to switch to.
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These houses aren't my proudest work, by a lot, but I moved in two town NPCs and two town pets, this should enable pylons to be sold in the store. Note that on DDU and GFB all town happiness is disabled, they should always sell a possible pylon. But interestingly, they aren't selling any pylons at all.
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My world
does have a functioning forest pylon. having one exist in the world does not turn off buying more from a store. It is currently set here in the world and has two townsfolk and a pet there.
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Finally, I tried picking up this existing forest pylon and accessing the pylon network from the house I built in image 3. The pylon failed, saying it is not in the correct biome to use.
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From what I know now, it looks like the forest pylon requires you to be a certain distance down in the world to work properly, and for that same reason it is not going to be sold in an invalid location that is too high up. You could probably make the pylon work by placing the pylon down far enough in your world but still close enough to the dungeon, as a way to access the dungeon via the lower sections.