Volume control not working properly in GUI

Awesome work & things just work out of the box (Airplay + wifi + amp+).

I am trying to control the volume from GUI, but it is having a very quirky behaviour. The volume starts from whatever the default value was, and allows me 1 step up & 1 step down (in value this is 2 steps though).

This behaviour is not present when I am using the audio jack as the output & happens only when the output is set to I2S (hifiberry amp).

Raspberry pi 2 model B
TP-Link WN725N wifi dongle
Hifiberry Amp+
Volumio 2.001

Thanks for your help.