Ask for help and report issues with the Mac OS X version of Transmission
Me too, after several frustrating months quitting and restarting Transmission ormaking sure I don't exceed a couple of simultaneous torrents, I'm now using qBittorrent. I don't like its interface and somehow it only downloads at a fraction of Transmission speeds, but at least I can let it running without any fear of my active connections to stop suddenly.
I wonder if this is actually a Big Sur issue. I'm still on Catalina and have notice recently that the chosen port gets closed after a while and Transmission grinds to a halt. If I click "Randomize", Transmission eventually finds an open port and resumes transfers. I gave up on a Little Snitch and uninstalled it several months ago, so that's not the problem. I'm using an AirPort Extreme with "Enable NAT Port Mapping Protocol" enabled. This is with Transmission 3.0. I did not see this problem on prior versions.