The bug

When unlocking explorer map trades with a Cartographer Villager, a region file is created for every chunk the game tries to spawn the respective structure. Each region file contains a single empty chunk and takes just a few KB. For the arbitrarily chosen seed "villager"; 30+ files are created when unlocking the Ocean Explorer Map, and 900+ files are created when unlocking the Woodland Explorer Map. If you are unlucky and the nearest woodland mansion is further away, you easily get 1000+ files.

How to reproduce

1. Make a creative world with the default world generator. Seed doesn't matter but I used the seed "villager" in the screenshots.
2. Open up the folder of the world you just created and go into the region folder. You should see just a couple of region files as you would expect. In my case there is 6.
3. Go back into the world, spawn a villager, place the Cartography Table and trade with him until he unlocks the ocean explorer map.
4. Now check the region folder again and there is a bunch of small region files. In my case there are now a total of 39 region files, most of which are just a few KB in size and containing a single empty chunk. This can also been seen in an attached screenshot.
5. Continue trading with him until you unlock the woodland explorer map. The region folder now has 939 files where pretty much all of them contain a single empty chunk. I have attached a screenshot showing the region folder in this stage as well.

I assume this is happening because the game is checking potential locations for a Monument/Mansion until it find a valid location, and the chunk and region file is created for each location it checks.

Unsurprisingly, using the /locate Monument and /locate Mansion commands has the exact same results as trading with the villager.

Some versions I tested that have this bug include:

• 1.16 Snapshot versions
• 1.15.2
• 1.14

1.13 has a less severe variation of the bug where only the region file where the structure succeeds at generating is created. Here the file contains a single otherwise empty chunk with some structure start info. This isn't nearly as bad as the newer versions, but still creates an unnecessary file.

1.12.2 doesn't have the bug. No region files are created with this version.