1.67 on Linux Debian 12 (stable) with KDE (Desktop) it is hashing the file which is currently being downloaded by the bittorrent and is incomplete.
That is useless and wrong. How to prevent that and why it is not prevented by default?
If I need to supply any data for debugging/reproduction, let me know which exactly.

Just to be clear, you have an incomplete torrent that fopnu is hashing?
Fopnu does not deal with torrents.