I was able to reproduce the issue with a bullseye debian-live with mate iso image and qbittorrent 4.2.5, but only with the icon style monochrome (both light and dark theme) selected in Preferences->Behavior->Tray Icon Style.
The icon is stretched vertically and it gets worse when enlarging the panel. The normal (blue) icon is ok and scales correctly when enlarging the panel. Other desktops (I tried plasma, xfce, lxde, cinnamon) are ok. Searching the web I found a report (and a workaround) [1] for a few other qt5-based applications that show the same defect in mate systray. I'd say it's likely an issue of either mate or qt5 but qbittorrent should be not guilty for it. Regards BZ [1] https://forums.linuxmint.com/viewtopic.php?p=2093352
