The story behind this is that when you import an NZB file, it sometimes download a movie file with an obfuscated file name.
This obfuscated filename will then be changed to the name of the NZB file.
NZB filename: movie_2021.nzb
File to be downloaded: 8das98d7a.mkv
The downloaded file will then be changed to movie_2021.mkv
However, if several mkv files are downloaded via the NZB, only the first mkv file will be changed and not all of them. If this first file happens to be the sample file, it will be changed.
However, this is not very common. Usually an NZB contains only 1 mkv file. You can therefore turn off this feature if you do not want this.