Prevent Radarr managing anything except video files?
When a file is manually replaced, for example after converting from an mp4 to an mkv; radarr decides to delete everything in that movies folder: posters, backdrops, subtitles, NFO files, leaving only the new video file; even though none of these were created or managed by Radarr ever.
This causes Emby to have to rescan/reidentify the item, re-downloading all the extra data, and it's now lost all custom metadata that was stored in the nfo, particularly the original date added to emby and it now has no subtitles.
I'm not changing the files via radarr, external programs are replacing the video file with a new one (usually from converting the original to hevc, mkv; replacing the original):
Have a movie already imported to Radarr; Manually delete the movie file from outside radarr, then add a new one to the folder. Then return to radarr and click 'refresh and scan' or have the scheduled task to scan files run.
Radarr recognizes the old file is missing and deletes all associated files (images/subtitles/nfo); then separately, sees the replacement video file as new and associates it with the movie for the folder it's in.
Ah, okay. Yes, I got the same results as you, even if you do it the opposite way, add a movie through Radarr, and it will replace the other movie and wipe the other data.
Through my testing, it does seem to only wipe it if Radarr thinks there's a movie there already. I don't think this solves your problem, but if you empty the movie folder, scan and refresh radarr, then add the data back, it gets picked up. It only wipes the data when it is replacing it, which is probably intentional because different files might have different metadata? I'm not sure.
Emby sees the new movie in its media folders and downloads posters, subtitles, and saves all metadata to an nfo.
Emby converts the new file to h265, replacing the original (this may be immediately, or much later)
Radarr sees the old file missing and deletes all the metadata Emby had put in the folder with the movie file.
Radarr sees the new file and associates it with the movie for the folder its in.
Before emby scans for the files radarr deleted, it's trying to serve them to users and failing as they dont exist.
Once emby does scan the files and remove the missing ones from its db, it has to redownload all the metadata again and no longer has the nfo file telling it the original date added, so the movie is moved to the front of the 'recently added' list as if it was a brand new movie.
This has been consistent across two seprate setups: my original windows 10 based setup, and my current debian-docker setup.