Volume fluctuating

I am new with Volumio. I was using Kodi, but with some things I found it slow and because I only used the music capabilities, I decided to try Volumio.

The first impression is very good! It runs smoothly, looks nice and seems to have the right features for me.

Till now I have only one problem sometimes.
If I change the volume through my Android tablet or phone, the volume keeps going up and down for a few seconds (sometimes up to a minute long!)

I have Volumio 2.246
I am using the official Volumio Android app (Android 4.4 and also Android 7)
I am using a Raspberry Pi 1B+ with the audio jack to my receiver.
I am using most of the default settings and mp3 music on a USB device.

Does anybody has an idea? After some seconds/minutes the volume stays constant…