MongoDB – changing the name of the data directory changes the data

I am witnessing a very weird behavior of MongoDB.

I run MongoD on a test system (Windows 10, 64 bit) like this:

"C:  Program Files  MongoDB  Server  3.6  bin  mongod.exe" --dbpath "C:  MongoData"

I then stopped the server and replaced the MongoData directory with a set of data from a production server. I start Mongod again, but I get a collection with 4 documents, which is what I had used to have before I have replaced my test data with real data.

This is not correct because I would expect to see loads of documents in a real data set. I then try dozens of things, including pointing MongoD to the data folder at its original location, which gives me all the documents from the actual collection. Phew, the data is safe. But what's going on in my test system?

I noticed that if I renamed the folder of my test system in MongoData2 and ligated on it, I recovered my appropriate production dataset. but if I rename MongoData and launch Mongodod by pointing to this folder, I get 4 documents again.

What's causing this?