
However if I shutdown SnakeOil and start it again then the mounted folder remounts

I have also test to first do an reboot (= it does NOT remount) and then a shutdown and a restart (= it does remount). I hope this can give some clue what the problem is.
NAS setttings
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Synology DS1512+ (DSM 6.2.4-25556 Update 8)
File Services / NFS
- Enable NFS: Yes
- Enable NFSv4.1 support: Yes
- NFSv4 domain: "empty"
Shared Folder / NFS Permissons / NFS rule
- Hostname: "IP of SnakeOil"
- Privilege: Read only
- Squash: Mapp all users to admin
- Security: sys
- Enable asynchronous: Yes
- Allow connections from non-privileged ports (ports higher than 1024): No
- Allow users to access mounted subfolders: Yes
Mont path: /volume1/Media
SnakeOil settings
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Music Library / Mount Points Configuration:
- Media: "NAS IP":/volume1/Media/Musik
- Mount Point: NAS
- Media Type: NFS
Exact the same settings has work in previous versions of Snakeoil and I also use this in my Linux Mint fstab ("NAS IP":/volume1/Media/Musik /home/robban/Music/NAS.Music nfs defaults 0 0) without any problems for many years.