The TV reports what it is receiving, not what is being sent out. 

There are two ‘types’ of Dolby Atmos, one carried by Dolby Digital Plus, which requires an ARC connection to your Sonos. The other is carried by Dolby Digital MAT, and requires an eARC connection to your Sonos, due to bandwidth. 

If this older TV doesn’t support Dolby Digital Plus on its ARC output, the Sonos will be reporting to the CEC computer in the TV what it can receive, and the TV will output the best signal it can (depending on settings, of course). 

It can pass Atmos at Double digital plus and does fine with the normal 4k fire stick and Sonos app says Atmos but with 4k max it only says dd plus 5.1

If your Sonos Arc is getting Atmos audio from the ‘same’ TV, using the FireStick 4K, but not from the FireTV 4K Max (when its connected to the same TV HDMI port and you’re playing the exact same audio/video source from the same Amazon account, then it points to being an issue with the FireTV 4K Max.

I would check its audio-output settings …and if still no joy, then perhaps speak with Amazon Customer Support.

That’s the odd thing is the tv is recognizing it as Atmos but the Sonos isn’t which makes me think the 4k max passes it in MAT like apple tv hence it wont work without earc. Why just trying to confirm if anyone else had seen it.

The purpose of the CEC computer in the TV is to accept data from the Sonos, indicating what formats it can receive. The CEC computer then tells all other connected sources, like the FireTV, what the appropriate signal that can be sent. Unless you have CEC turned off on the TV set, or the FireTV, it is supposed to ‘just work’, the reason why CEC exists. 

I would be getting in touch with Amazon to figure out what is going on. 

Hello I just wanted to jump in here and say that I have had the exact problem as you. Normal Fire 4K Stick outputting Sonos app-certified Atmos through ARC to my Arc with no problems using Apple TV and Disney+ apps. I buy the Max and theoretical Atmos signals come up as 7.1 on Sonos app and sound is choppy and distorted. I change audio settings on Max to Dolby Digital rather than Best Available or DD+ and all is well but obviously no Atmos. It must be a Max issue if your TV doesn't have EARC so I think I'll be sending the Max back. Funny how an upgrade isn't an upgrade in this instance!