In a Sonos stereo pair it is always the left speaker that runs the show for the pair,  The right speaker just plays what it is told to play when it is told to play it.  Often Sonos will use “direct routing” so that the data packets are sent directly from the left speaker to the right one, rather than via the WiFi network.  Even if a problem occurs in getting the data to the left speaker, it may be the right speaker that stretches things too far

I don’t believe there ia a software bug.  If the speaker is dropping out it is more likely to be wireless interference or other network latency, or something environmental.  Whatever you may think.

What WiFi channel does your system use?  Have you tried changing it?  How far apart are the speakers ?  Any other wireless stuff such as baby monitors or wireless security? (Sorry if you don’t like the voodoo.)

FWIW I have a stereo pair of Era 100s that have run without problem for nearly a year.   I also have a pair or Era 300s, for which the same applies.

thanks for the information 

Overtime, I have systematically tried all channels available - on three different networks. The speakers are less than an arms length apart. I also have some Sonos one that are rock Steady.
 

What I mean by voodoo - is if you have the same fault occurring on three separate networks, the detailed logs show no network problem and no other client devices have any problems, the only thing left is the Era 100. 

OK you have done what troubleshooting you can.  I think the best next step would be to submit a system diagnostic immediately after the drop occurs, note the ref no, and call Sonos Support.  I recall a recent case in which this revealed that one speaker was connecting to 2.4GHz and the other to 5GHz, and this router didn’t bridge the bands.  Just as an example.

I don’t think the comment about other devices being fine proves anything.  Other devices don’t have to keep in constant touch over the network with other devices, including controllers.

But I do hope this gets sorted for you.