qBittorrent official forums

qBittorent always download all the files no matter if I uncheck them.

This feature simply not working.
Last tested on v 3.3.10

Just tested on 3.3.12

Same. qB download all files and simply ignore user selection.

Works fine on Win10 3.3.12.
"At the moment qBittorrent dumps the unselected files that contain cross-file-pieces in the same directory as the desired files. They aren't downloaded fully as you may think, only the cross-file-pieces."
This has been brought up a few times on Github, e.g. https://github.com/qbittorrent/qBittorrent/issues/5379

E.g. a recent partial torrent gives me 3 !qb additional files for 2 actually downloaded files.
!qb files: Size 4.42 GB, Size on disk 3.82 MB

Ah, right. Enable append !qb to incomplete files in settings otherwise I guess it can look like they are actual files.

Yes, I understand what are you talking about.
However, files are downloaded fully (I can open it without problems and it uses space). For instance, I just downloaded video clip where I only selected .avi file, but all other folders, readme.txt and other files are unselected. After download completed all files are there, including readme.txt, folders, screenshots etc. and I can open all files. That's weird.

However, now I checked option Append .!qB extension to incomplete files  and will see results.

You can expect to get LOTS of unwanted files if the piece size (4 MB?) is a lot bigger than the little (*.txt) files.