I'm not sure, but if I remember correctly, the problems do not happen with Android 8, all started with Android 9.
The issue is pretty simple: when I link the left and the right Zolo Liberty earphones together, the Android volume level popup appear for every single sound (keyboards taps, notifications, etc).
The issue doesn't happen when there is only the RIGHT earphone linked to the smartphone.
Useless to say that this problem is very frustrating because the popup appear very often and you can easily tap on it wrongly.
Smartphone: Google Pixel 2
SO: Android 9