Airpods pro volume goes up when using Spotify on iPhone

When using my AirPods Pro and iPhone 11 with the Spotify app, the volume changes significantly (it always goes up, never down) without me adjusting anything, or even touching the iPhone (Siri is disabled).

This does not happen:

• with any other apps on my iPhone
• if I use Spotify from another device (e.g. the web player using Microsoft edge)
• If I use my iPhone & Spotify but use non-apple headphones

It only happens when I use Spotify on my iPhone with my Apple AirPods Pro!!!

The cynical side of me says this is Apple trying to dissuade me from using Spotify and take up their multiple and persistent requests to sign up to Apple Music by making me think there is a glitch in the Spotify app.

Its not like they have a track record of making customers think things are failing when they are perfectly OK (e.g. by reducing the iPhone battery performance to make people buy a new iPhone)!!!

Has anyone got any experience of this?