I am wondering if you have two related-but-not-identical issues -
On the Mac, I’m not sure if automatic scanning of network shares works. If it does, it also relies on support from the NAS. (I’m not optimistic. I suspect it would require a different mechanism + polling. Maybe somebody can speak up.)
And on the NAS - what is the NAS, by the way? Synology? - you might be running into a limitation with the maximum number of directories that can be monitored automatically.
Generic Linux, plus some background information:
https://forums.plex.tv/t/increase-the-number-of-directories-linux-can-monitor-notify/209156
If Synology:
Either way, it’s worth following those instructions to identify how many directories are in use, and if applying this fix would be worthwhile.
