I do not remember there is a .parts file in the past, and it is not deleted automatically.

FDM should not create any *.parts file, but qBittorrent lib used in FDM 5.1 may leave such files from unfinished torrent downloads. See explanations here: https://github.com/qbittorrent/qBittorrent/issues/8175

You can also see *.part files when you are downloading files with your browser (f.e. Firefox or other Gecko-powered browser).