This probably happens when you group the 300’s with any non-Atmos speaker. Which would mean it also happens with the 100.
When speakers/rooms are grouped, they will all play to the “lowest supported format” of the speakers in the group.
Thanks for your responses, I thought that with the newer technology speaker the signal would be able to pass through?
Thanks for your responses, I thought that with the newer technology speaker the signal would be able to pass through?
Pass through to what? Since all rooms are playing the same stream, if a room in a group is incapable of playing a particular format, then the stream is downgraded to accommodate that room.
When speakers/rooms are grouped, they will all play to the “lowest supported format” of the speakers in the group.
Not quite, otherwise they would all play in SD.
The group will play the highest supported format of the least capable speaker, which in this case appears to be Ultra HD.
When speakers/rooms are grouped, they will all play to the “lowest supported format” of the speakers in the group.
Not quite, otherwise they would all play in SD.
The group will play the highest supported format of the least capable speaker, which in this case appears to be Ultra HD.
Apologies - that’s what I was trying to say but my attempt came out quite wrong!
“Highest common denominator” is the fancy way of saying it 
This bit may make the Era-100 Atmos question a bit less confusing as it does support Atmos when used as a surround but not alone or in a Pair.
https://www.sonos.com/en-us/shop/era-100-black
Can I experience Dolby Atmos with Era 100?
As a standalone speaker, the acoustic architecture of Era 100 does not support playback of spatial audio content. For those who want to enjoy content in Dolby Atmos, we recommend Era 300.
When paired with Arc or Beam (Gen 2) as rear surrounds, two Era 100 speakers will support playback of Dolby Atmos content.
