I think based on the example you need give full protocol line
<pathsubstitution> <substitute> <from>nfs://rrl-nas/</from> <to>nfs://192.168.3.21/</to> </substitute> </pathsubstitution>
Right. I knew that. I just didn’t want to type all the other parts of the XML. Just the parts that would be different from some other setup.
OK… So here’s another tidbit of data; this is not an OSMC issue at all. It’s a Kodi issue.
I am able to duplicate this on my Kodi 17.4 install on my laptop running Linux Mint 18. All the network config is the same as the OSMC boxes (Vero 4k and Pi3) When I try to scan for new content on the TV show title, it fails to find the episodes, just as described.
There are still possibilities on this; around the time that I noticed the problem was when I switched from SMB to NFS. I’ve been using hostnames for ages with SMB with no issues like this. So my next test will be for me to switch the file access back to SMB from NFS and see if that has any effect.
As I mentioned early on in this discussion, I’ve also seen the problem on Mint 
It would be interesting if SMB makes a difference. All my shares are NFS.