I have this same problem with ERA300 for surrounds.  Using test files, the rear surround tests were not sending anything to the ERA300s but rather played from the Arc Ultra.

A short update after further tests with various test videos: Unfortunately, the noise for rear left and rear right is actually louder from the Arc Ultra than from the Era 100, whether without Trueplay, with Trueplay or with Trueplay Advanced (now checked again with my wife's iPhone).

However, the problem only seems to occur with TV audio. I have now played a test audio via Apple Music and surprisingly: The sound for rear left and rear right really only comes from the Era 100 and not additionally from the front!

This thread cannot be marked as solved yet, there are many of us with the same problem

I don't know exactly why, but my last post was deleted or not even displayed. I have noticed that a test song in Dolby Atmos from Apple Music does not cause any problems. Here the rear sound only comes from the Era 100...

 I have read that Dolby Atmos sources play as expected where rear channel sound is only played through the rear speakers and that only 5.1 non Atmos sources have rear channel sound, that should only be played in the rear channels, also being played in the Arc Ultra.  Is this your experience?

No. My test sound videos are Dolby Atmos sources. It also says in the Sonos app that it is Dolby Atmos. Nevertheless, the sound for the rearmost rear speakers also comes from the Arc Ultra.

I have just written to a ‘Sonos employee’ in the chat. (German) I was told that this is not an error and that everything is working absolutely correctly. Here is the entire chat history (automatically translated):

Me:

I've been using the Sonos Arc Ultra + 2 Era 100 as a home cinema setup for a few days now. I tried out some test videos for Dolby Atmos and realised: Strange. The sound for rear left and rear right comes not only from the Era 100, but also from the Arc Ultra. The sound from the Arc Ultra is even louder than from the Era 100.

I have already tried setting the system to Trueplay, Advanced Trueplay or no Trueplay at all. However, the problem still occurs.

When I start an Atmos test song via Apple Music, everything is output correctly. Here, the rear sound only comes from the Era 100.

Unfortunately, setting the level of the surround speakers in the Sonos app doesn't help either. It does come out louder from the Era 100, but also from the Sonos Arc Ultra. Is this a known bug that will be fixed? Or is my Arc Ultra defective?
 

Sonos-Employee:

Hi, the Arc Ultra is working as intended in this case - it will output audio in a surround/DT configuration. You can adjust this configuration with EQ settings or possibly Trueplay as you have already tried, but there is nothing wrong with the Arc Ultra's output. 

Me:

For what reason is the rear audio on Apple Music only played from the Era 100, which is correct from my point of view. And why isn't it the same for TV audio? It can't be that the rear sound from the front comes out of the Arc-Ultra louder than from the rear. That makes no sense at all. 

Sonos-Employee:

It does, because by itself Arc Ultra is the first 9,1,4 sound bar. If you add a pair of ERA 100* or ERA 300 as surround speakers and sub(s), it would create up to a 9.1.4 system with 9 ear level audio channels, 1 subwoofer channel with 2 height channels from Arc Ultra and 2 more vertical channels from the ERA surround environment.

* Era 100 has virtualised height channels.  This allows the Arc Ultra to function in the configuration in which you have set it up. This changes when playing Apple Music, as the content from the TV is different from the music service content altogether. 

Perhaps I can interest you in a feature request to have this changed in a future update on the Arc Ultra? 

Me:

Do you provide your own test videos that I can use to check the function of my system? If this is really the case, then I will return the Sonos Arc Ultra. In my opinion, this is not a feature but a bug. 

Sonos-Employee:

We do not currently have this material on hand, but if you would like to proceed with a return, you are welcome to do so at the time of purchase. Thank you for your feedback anyway. 

its the same issue with Atmos or Dolby 5.1 for me.
Sound that comes out of the rear surround sound comes out also from the ARC Ultra (and at a louder volume)

 That’s a deal buster.  I’ll wait to see if that issue is addressed.

This comment is PURE speculation. Could it be that the Arc Ultra is more advanced than the Arc and capable of doing a lot more FX processing as a stand-alone unit. It therefore recognizes when Era 300 ‘s are used and says...”OK Era 300 you can handle what I’ve done on my own but now since you’re just as capable I’ll send the material to you”. The reverse would apply when the Arc Ultra senses that Era 100’s are being used and therefore keeps more of the FX processing upfront. As I said just PURE speculation.