It has been a long time since I used stremio, but IIRC it used to have a max cache size for torrent plugins. The default setting was something like 10GB, which means older torrents eventually get deleted after you stream a certain amount of other content.

I suspect stremio seeds torrents in the cache directory, so if you increase cache size, you can potentially continue seeding torrents for more time after the initial download.

It would be cool if this was true, but no, Stremio does not seed cached files.

https://github.com/Stremio/stremio-features/issues/626

Somebody made an extension that adds cached torrents to your local qbittorrent instance though, which is a nice alternative.

https://github.com/Vance-ng-vn/Stremio-Seeds

Note: I haven't used this, since I use Stremio from a Chromecast.