I was able to systematically reproduce the bug in stretch (qbittorrent_3.3.7-3+deb9u1 / libtorrent-rasterbar9_1.1.1-1+b1). I upgraded the latter to 1.1.4-1 (from snapshot.debian.org) and the bug disappeared.
So likely the bug affected the package libtorrent-rasterbar and it is fixed now. Regards BZ
