Guardian app volume issues

[MystrDVS]

Hi, the volume on Spotify has started decreasing once I click on a story, in the same way it decreases when you play a video. This doesn't happen on the Guardian main page.
I have been using Spotify alongside the Guardian app for many months with nary a problem. Is anybody able to tell me if this a Spotify issue, Android issue or Guardian issue?

My Spotify version is 3.4.0.764.
Android 5.0.1
HTC Sense 6

Many thanks!