The history of thumbnails (or just a previous thumbnail) is embedded in an image file?

Jpeg files can contain Exif data, which can include a thumbnail. Theoretically, it seems it would be possible to store more than one, but I don't know of any program that does it, nor of a program that would read it.

What is likely to have happened in your case is that the program you used to crop the image modified only the main image, without modifying the embedded thumbnail.

exiftool can be used to read or replace Exif data, including the thumbnail. As show in the documentation, exiftool -b -ThumbnailImage image.jpg > thumbnail.jpg can be used to extract the thumbnail from a jpeg, exiftool '-ThumbnailImage<=thumb.jpg' dst.jpg to replace it. If you want to completely remove the embedded thumbnail, you could use exiftool -ThumbnailImage= dst.jpg.

Note that exif data can contain much more than a thumbnail, as exiftool -a -u -g1 a.jpg will show you.