Here it is: https://documents.westerndigital.com/content/dam/doc-library/en_us/assets/public/wd/product/external-storage/my_cloud/my_cloud_home/user-manual-my-cloud-home.pdf

Fascinating. I’d have assumed it would be in the settings that are explained starting on page 18 of that doc, but there certainly is no confirmation that it exists in that menu. Most odd, and thank you for posting that link. 

I hope someone from Sonos has greater familiarity with that particular device, and is able to assist here. I’d also be tempted to reach out to Western Digital for more context, but that’s me. 

Been talking on the phone to customer service in NL for almost 2 hours. The guy on the other end of the line was very friendly and helpful. He had me change the ethernet wiring of my system, rebooting the NAS, rebooting the router, re-indexing the music library (manually and automatically), all to no avail. He succeeded in reproducing the issue by remote control twice. That convinced him there was a real issue going on that he couldn't solve. In the end he escalated the problem to someone higher up in the technical chain. To be continued. I'll keep you informed.

Thanks @beynym - it’s a real unusual issue.. I don’t think I’ve ever come across it in these pages before. I’m surprised some others have not (yet) reported the matter, but it’s good to hear that it was reproduced by the Support Staff accessing your setup remotely and have now escalated the matter for investigation. 
 

Also, thanks kindly for the feedback here and hope the matter gets fixed for you soon.

If you power down PORT do you have the issue with another player?

I powered down the Port, it disappeared from the system. Then I put a Sonos Playlist in another room and triggered an automatic music library update. Instantly yet another room vanished from the left pane of the Sonos Desktop Controller, not the one that was playing the music, but the one the SDC was associated to. Before powering down the Port, the SDC was associated to it. Probable conclusion: an automatic library update kills the association between the SDC and the room. It also deletes the active queue in that room.

Today the NL Sonos helpdesk told me they’re convinced that the culprit of the problem must be found in me using unmanaged switches in my network. They want me to buy and install managed switches. I don't think they're on the right track and I've told them as much. I've been using unmanaged switches since the dawn of times, I mean since I own Sonos, since 2010. Never had any problems, only since the introduction of 13.4.1 my problem started to occur.

I told them what happened when I powered down the Port (see the message above this one). Am awaiting their response, will let you all know how this develops.

Good grief. It sounds like they could do with a basic primer on the OSI model.

Since it appears to be the AssociatedZP which is getting into trouble -- which is the node doing the indexing -- have you checked all your network connections are healthy? It’s just conceivable that a connection struggling under the indexing load might get into such a state that a soft reboot could be being triggered. 

I've powered down and restarted my two unmanaged switches and then scheduled a library update. Lo and behold: no more disappearing queue. Let's wait and see what happens in the next couple of days.

On the same or a different AssociatedZP? If it was a scheduled background update it would be hard to know which node executed the scan, without examining the full diagnostic. See how things go anyhow. 

It was on another. A couple of minutes ago I repeated the procedure (scheduled automatic library update) with another AssociatedZP and to my surprise the Port (not Associated!) disappeared from the Sonos Desktop Controller. So maybe the problem doesn't lie with the AssociatedZP, but just with the Port - whether it's Associated or not. So I factory reset the Port, to be able to start from scratch.

Just now another scheduled automatic update, with music from Sonos playlist on all players. The SDC was associated not to the Port, but to another player. When the update launched nothing happened! So all good for now. Maybe it was the factory reset of the Port that did the trick. I don't know. I'll keep testing the coming days.

The problem still persists.

This morning yet another test. The Sonos Desktop Controller was associated to Room A, not being the Port. Room A and the Port were playing a Sonos Playlist. A scheduled automatic library at 6.40am made the Port disappear from the left pane of the SDC, in Room A nothing happened. When the Port re-appeared, the queue was empty.

Diagnostic number: 935830396.

I'd appreciate it if someone could take a look at the diagnosis.

Update for those interested:

Been in contact several times with the NL helpdesk. Sent several diagnostics, the problem persisted.

The Sonos guys pointed me in several directions: firewall, managed vs. unmanaged switches, connecting the Port directly by cable to the router, connecting the Port via Sonosnet to the Boost, reducing the number of music files to index, changing the name of the PC's in my household, re-installing the app, changing the SMB from 3 to 1, removing the TV, printer and/or settopbox from the network - nothing helped. The problem persisted.

Finally they suggested the cause might be hidden in network connectivity, causing a network delay in transmission speed. In the diagnoses they found remarks leading them in the direction of 'host down' or ‘access denied'.

My diagnosis is they were clutching at straws. Anyhow, the problem arose with the introduction of 13.4.1 so maybe I'll just have to wait for the next update.

BTW: why did none of the Sonos guys monitoring this community bother to react or even look into my diagnoses?