Sending audio via Airplay from ATV4K to Airport Express and on via Toslink to external DAC

My AVR's built in DAC is too harsh for music (but good for video). I recently discovered you can have turned on both HDMI and Airplay to an external device from Apple TV 4 and 4K. A device which in turn can send Toslink to my better external DAC! In my case, Airplay to an Airport Express and Toslink to a Modi 3.

However, I cannot seem to get Spotify to work. On screen, in Apple TV, the app works fine with just HDMI (to the harsh AVR DAC). However, when I turn on the Airplay option, it goes silent for Spotify. Other music sources work using this method like internet radio via Siri, etc. Of course, all video is fine with audio as the 4K knows to send that on to the AVR via HDMI. That combo, a kind of automatic switching, is really nice. If only I could get it to work with Spotify…

It is not my AVR as, it is out of the mix (other than as an amp via RCA analog via the Modi 3). It has to be some Spotify or ATV limitation or setting I am missing. I also tried different combos in the audio settings for the 4K but, none make any difference. Indeed, left to default, Apple internet radio works great from the 4K to the Express to the Modi.

Does anyone know what is happening? Why does Spotify not work?

Before you toss me off to Spotify; 1) they have no tech support, none 2) this might be an ATV4K setting I am missing or set up wrong.

BTW: My AVR has no pass through for PCM via Toslink or coaxial. And, of course, the Apple TV 4K has no Toslink. It is but a dream…

BTW, BTW: the one Toslink in the AVR is being used by the TV OTA receiver as the set is old enough that it has no ARC via HDMI. But that, again, is just FYI and has no bearing as, that would just be sending the audio to the harsh AVR DAC. Not my objective.

BTW, BTW, BTW: These are all ethernet on home network. Wi-fi not involved. Even turned off on Airport Express as it just messes up my main router and other devices. But, again, should not matter.

Literal back of envelope sketch: