Ok just wanting to lay out all the observations we have so far.
Affected Environment:
- MacOS 15
- Synology NAS
- One NAS or multiple NASs
- SMB mount
- One mount or multiple mounts
Behavior:
Some files go missing after a library scan
- in some cases it’s an entire library goes missing (entire share)
- in some cases only specific number of files go missing (not the entire share)
Potential work around
Disable any and all automatic scanning options for now.
- Settings > Server Settings > Library
- Disable
Scan my library automatically - Disable
Run a partial scan when changes are detected - Disable
Include music libraries in automatic updates - Disable
Scan my library periodically - Save
It has been reported that if you readd the same path to any of the affected libraries the missing files should be restored.
- Select Edit Library
- Select Add Folders
- Select Browse for Media Folder
- Select the same existing folder
- Save
I would like others to confirm 
temporary work around. Obviously if you are adding new media to these libraries and want them scanned in, you might need to jump through these hoops a couple times in order to get them in.
Another data point to have from any affected users here, is your SMB settings from Synology.
