That seems odd. Does it work when you stream another source? I’m assuming the Sub mini is ‘bonded’ to the Era 100s, and show up in the same room. 

From what I see in the app, it is bonded.

Agreed, that answers one of my questions. 
 

I got the same results streaming from Spotify and Amazon Music. 

Interestingly, if it is with one Era 100, the Sub Mini plays audio.

All are on WiFi.

Ok, so then we know it isn’t related to Spotify, which is useful data. It is even odder that it works with one Era 100, but not a stereo pair. I’d suspect a software issue at this point, assuming you’ve rebooted / power cycled all of them, so I’d recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. But don’t post the resulting diagnostic number here, they get sensitive about GDPR. It would probably help a lot, since this seems to be the only case reported, that they get the hard data. 

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Ok... So... It works now.  

As advised, I contacted Sonos Support, who was outstanding, knowledgeable, and thorough. 

Due to my place's layout, I use network extenders (which are not completely meshed but are on the same WiFi network).  

The SSID is the same across my WiFi network. 

The stereo-paired Era 100s were on the router's Wi-Fi, and the Sub Mini is connected to the extender's Wi-Fi.

Since all Sonos speakers are on the same network, the app could find, connect to, and bond the Era 100 stereo pair and Sub Mini.

However, Sonos speakers are not supported for use with WiFi extenders.

Not supported doesn’t necessarily mean they don’t work, but merely that Sonos can’t troubleshoot if you run in to issues.

Many/most extenders create a different subnet than the original, I suspect, while maintaining the same SSID. This breaks Sonos’ ability for the speakers to communicate with each other, as well as the ability to communicate with the controller. 

I’m glad Sonos Support was able to resolve your issue, as that was the first mention of extenders in this thread.