I discovered the culprit, as explained in this comment:
<https://github.com/qbittorrent/qBittorrent/issues/13688#issuecomment-995201409>
It seems that, due to a Qt bug, setting QT_SCALE_FACTOR_ROUNDING_POLICY
to PassThrough makes qBittorrent ignoring fontconfig parameters.
And qBittorent made this value the default, apparently to fix issues
with HiDPI rendering, primarily on Windows:
<https://github.com/qbittorrent/qBittorrent/pull/12273>
Reverting the commit a9b0d84df95db7c30c668190ccac00d37cbf1b40 completely
fixes the issue for me. I can also set the env var to anything but
PassThrough as a workaround.
