That is not a Apple Spacial Music file that your being served! It's just a stereo file, and you have not enabled lossless quality in your settings, or ATMOS playback. Is suspect that is not the MacOS version of Apple Music?
Yes, it was not, I did repeat your steps on my Mac and had similar results.
Confusing I agree. I think we are primarily different use cases, e.g., I was using a preview of Apple Music on Windows to send music to a DAC and headphones, and it came through as the file you saw (now below on the right), however when I downloaded the file, we've been looking it it came down as lossless and I played that. It is on the left, the streamed version, which is lossy is on the right (AAC) vs HLS, the encoded ALAC is on the left.
So it appears that what get may not be what the label says, unless you have the correct equipment.
Atmos on any streaming audio service is lossy!
Not sure what "Atmos" means in this context; e.g., when I play Apple Music on an Apple TV and send it to an ATMOS capable sound bar I get 10 Channels of Dolby TrueHD ATMOS (image on the left below). Since Dolby TrueHD is lossless is the whole payload (Dolby TrueHD stream + the ATMOS bit) lossless or does the ATMOS bit come in lossy or lossless versions or is it simply embedded in the lossless stream? On the right is my iPhone over AirPlay to the soundbar which sees 2 channel PCM (right image), I don't know if its lossless or not.
Finally, all the speakers in my house are to playback lossless ATMOS, that will be a bit more work to find out what is actually happening with different devices.