Paired (original) HomePods change volume by itself

I have two (original, first gen) HomePods. They are paired as stereo-pairs. Unfortunately since a few weeks, not sure when and how it started exactly, they are changing volume on the most random times. This really frustrated me and I have no clue how to solve it.

Anybody else having the same issues? I saw this thead: https://discussions.apple.com/thread/252127091. It's "locked" due to lack of recent activity. But my problem still exists!