The file was post-processed, moved to the mount, and then Plex would have started scanning it immediately. I don't believe there would have been another write operation in there, but I can't be sure.
I actually did just cause this again with processing just a single file, so there does seem to be something strange happening here with this config. I'm going to try this with a test mount with debug logging on, but won't be able to do it right now. I'll report back with full debug logs in a few hours hopefully.