Volume level changes between apps - lingers on change

[cannibalistic_browny]

When switching between full screen gaming apps to others such as Discord or Chrome, the volume levels change a fair bit. Same when opening up a new full screen app. 
For example, open up a game that jumps into cinematic intro music. Quite loud for maybe 3-5 seconds then drops down maybe 20%. Stays this way until I alt tab to another app, which boosts the volume back up. 
Alt tab back to game, keeps that volume for 3-5 seconds before switching back. This can be quite loud indeed if I've adjusted for the game.
I've checked around the laptop and can't seem to find any audio profiles that would something like this.

I'm running the ASUS ROG Zephyrus G14 GA401IV (2060 card, animematrix model)
Win 10 Home 64-bit

Latest BIOS Version being used GA401IV.212
Checked through Armoury Crate - ROGS flavour of "useful" gaming optimizer - too and couldn't find any settings that mess with audio. 
 

[cannibalistic_browny]

So what are the rules on bumping in this forum....