Speakers disappearing from app, but not from the controllers on my Mac or PC

Every other day, some of my speakers disappear from the app. It’s not always the same speakers, and checking with two phones might sometimes show different parts of the Sonos system on each phone. The speakers usually reappear after a while (or after power cycling them). But strangely, even when my phone is not showing any speakers at all (yes, this also does happen), I can always see the whole system in the controller on my Windows PC or my Mac. Apparently, the speakers are connected the whole time, for the following reasons:

• Windows and Mac controllers can always see and control them.
• I can ping the “missing” speakers from any device on the network.
• My access points report uninterrupted connectivity to the speakers with great connection quality.
• The speakers keep playing even while the app doesn’t see them.

My setup:

• 5x One SL + 1x Move (all spread across the house, on different floors)
• Ubiquiti Unifi network devices
  • USW Pro 24 (main switch)
  • Various access points (Flex HD, U6+), all wired to the main switch
• I’m not using SonosNet since the speakers are too far apart and the materials used to build my house (and the floor heating system) prevent radio waves from spreading far enough. My WiFi access points provide great coverage, though, and every single Sonos speaker is located within one to three meters distance with direct line of sight to the respective access point.
• All speakers have static IP addresses and are on the same subnet as the phones and the Windows and Mac computers (which have static IPs as well).
• The access points with overlapping coverage areas are using different channels, but share the same SSID.
• The Unifi controller is forcing each speaker to stay on its access point (the closest one to the respective speaker) instead of hopping between access points (if it can hear more than one), so the whole network topology should be totally static. The only exception here would be the phones hopping from access point to access point while we move through the house. The Windows PC is wired to the main switch, the Mac(Book) is using WiFi.

After reading loads of similar posts and trying all the different “solutions” and approaches like power cycling speakers, restarting routers and access points, reinstalling the app, checking for WiFi interference or IP address related issues, I’m out of ideas now.

I also don’t understand the dynamics of this issue. There doesn’t seem to be a general problem with my setup because it works most of the time. It is unclear what makes the speakers disappear temporarily.

One thing that just came to my mind while writing this: I’ve noticed an “interesting” behaviour the last time this happened. The app on my phone was again showing only some of the speakers while the Windows controller was showing the whole system. I then used the Windows controller to group one of the speakers that the app was still showing and of one of the ones which were missing in the app. As soon as the “visible” speaker joined the group, it disappeared from the app on the phone. I was able to repeat this with all visible speakers, making them disappear one after the other until the phone was showing an empty system. Force closing and reopening the app still showed an empty system, but when I tried the same again a few minutes later, the app suddenly was listing all speakers again.

So, any ideas?