.partial file is the movie being moved (copied) to movie folder. You can watch the partial file get larger by refreshing folder and checking file size or using command: ls-alh (on Linux). After the copy completes it renames it and removes the .partial. Check finished downloads is running because it is copying the newly download movie file over as a part of its process. Continues to run until movie copy finishes. Depending on file size, can take quite awhile to complete the copy and use lots of CPU
I'm not seeding after downloading, so not sure why this is happening.
The use case for me is I run everything on a single machine.
Mergerfs allows to combine mount points and use policies so I use a policy to always write to my local disk and each night I upload to my Google Drive.
Mergerfs supports hard linking which is the primary reason that I use it.