ND5XS - volume changes

I have an ND5XS (not 2) feeding a non-Naim amp. Occasionally, the volume output changes with my phone when using Spotify through the Naim app. For example, if I open a video on instagram and I lower the volume on my phone, the volume will drop on the ND5XS.

I see no volume control in the app or in the ND5XS settings.

I would like to fix the output from the ND5XS.

What am I missing?

AFAIK the volume output is fixed to line level on the ND5XS. Volume control is provided via Naim automation to a Naim amp, so will only be available once enabled. I can only think the volume level change is coming from further up the chain.

I wondered about that, but in the Spotify app settings “Playback” is not enabled “when listening on another device”. It’s the iphone volume buttons that affect the output on the ND5XS.