When I play games alone (Skyrim, Elder Scrolls Online, Minecraft, Civilization) I usually listen to music at the same time. I've noticed that when any of these applications is making any noise, the volume output by Spotify is reduced very slightly. It's not the reduced volume for communication activity.
I know that the volume only reduces when other apps make sound; for example, when I fast travel in Elder Scroll Online, the volume cuts, and Spotify's volume increases, then decreases when I load back in. In Minecraft, when my character is standing still and nothing else is making sound (the volume mixer reports 0 sound output), Spotify volume is reduced. Interestingly, it appears that when no sound is being made by other apps, but my focus window is another app, Spotify is actually louder than it is by default.
My operating system is Windows 10.
EDIT: This does not happen when sound is playing from my browser. Only for games.