I have the same issue on my side:
this is my .plexignore file located in /volume1/Backup/Foto/Libreria Foto.photoslibrary/
# Ignore raw files
*.cr2
*.arw
*.ARW
# Ignore some photo folders
1999/*
2003/*
2004/*
2005/*
Albums/*
Apple*TV*Photo*Cache/*
Attachments/*
Before*Restore/*
Caches/*
database/*
iPod*Photo*Cache/*
Masks/*
Plugins/*
private/*
resources/*
SlideshowAssets/*
Originals/*
but files like
/volume1/Backup/Foto/Libreria Foto.photoslibrary/Masters/2019/06/23/20190623-163252/_DSC5104.ARW
are listed in photo library.
Plex Media Server v1.16.1.1273-4b6101f87 - Synology DS916+ x86_64 - build: linux-x86_64 synology - GMT 02:00
Linux version: DSM 6.2.2.24922-0, language: en-US
Processor Intel(R) Pentium(R) CPU N3710 @ 1.60GHz
/var/packages/Plex Media Server/target/Plex Media Server
