Using a NAS for original storage almost works fro mmy Mac (12.5.1), but if Lightroom (5.5) ever decides the drive is "disconnected" for whatever reason, there's no way to fix it.
Using this basic configuration:
if I disconnect and reconnect, I end up in a permanent broken state telling me that volume is offline and to "Please reconnect to finish syncing." The volume is connected and, in fact, just hitting that Browse button and saving again will convince Lightroom to reuse the same path, but it'll begin by moving all of my photos into it (not entirely sure what this means, but it takes a very long time).
I'd assume, if nothing else, restarting Lightroom would see the path I have configured and begin using it again, but it seems to permanently fail if interrupted.
Yeah this is a longstanding problem with Lightroom Desktop (the cloudy version). It really doesn't like local copies of originals to be on a network or external drive that could disappear. It is basically designed to work for an internal hard disk. Classic supports having originals on a NAS just fine and doesn't freak out when the network drive is offline.