I've had an issue of transmission completing torrents that are marked as partial. I'll have a torrent that is set at 30 or 40% for example, and suddenly it appears as 100% one day.
Devuan Jessie. ARM based CPU. Transmission from package repos, stable. 2.84.
This appeared recently. What I suspect is that I had set transmission to stop and start in crontab over the middle of the night in order to run other applications on the hdd, as the cause of this error. Only certain torrents are being completed, not all of them. It seems peculiar. I've decided to use alternate speed limits instead. And remove the service stop / start from cron, leaving it on 24/7. I will update this post if that doesn't resolve the issue.