Huh. I was able to run Plex alongside Jellyfin on my library no issues.

Just depends on what naming conventions you use.

For foolproof matching, adding Plex metadata in the style of "{tmdb-12345}" to the directory name would cause problems for Jellyfin, as they used to demand different formatting, for instance square brackets.

It has taken a years to convince the maintainers that this change is a good idea and not "letting Plex determine the future course of Jellyfin."