Path matching in Caddy is exact, so /qbittorrent will only match exactly /qbittorrent. What you need is to add a * at the end to match every other subpath, i.e. /qbittorrent*.
That said, be aware that you may run into issues when proxying to apps using subpaths. See the below article for an explanation. Generally, I recommend using subdomains for each service instead.