I'm currently using the nightly build #6683 on macOS 10.13.6 (17G11023),
I've been using transmission for nearly a year on this machine. The whole time transmission would work normally for about an hour or two, then at some point it stops downloading, uploading, adding torrents from the watch folder, and updating the tracker. On one site, I keep getting Hit and Run warnings., because the tracker thinks I'm offline.
If I quit transmission, and restart it, everything goes back to normal, including the H & R warnings going away.