LR changes RAW to JPG after update.

Or that folder could be a library previews folder?

As outlined, not possible. Adobe treats raw as 'read only'. Raws can be used to produce a new JPEG document (or TIFF) indeed. The raws remain raw unless the user deletes them otherwise. 

Most of the time, a person insists he never shoots JPG, and yet the problem is that the person's camera was accidentally set to shoot JPGs. Really, that's the only explanation, LrC does not turn RAWs into JPGs, its not even possible.

Technically, this is not possible. A raw file cannot be converted to a jpg, but it is possible to render a new file from the raw file (like when exporting), and this new file can be a jpg.

Additionally, LR never changes the original files, so I think something else is going on.

If you open one of these files in Develop, what are the latest steps in the History panel?

Could it be that you have exported some jpgs, and inadvertently checked Add to this catalog in the Export dialog?

That would import the jpgs after exporting.