Volume turns down by itself

I use Sonos in a restaurant and we manage the music + volume through the app.

Every day, at the same time, the volume turns down on its own. I turn it up, and 2 or 3 seconds later, it turns back down again. It seems to be capped at 15, won’t stay any higher. Later in the night (several hours later, once the restaurant is closed again), we can take it back up.

We have 2 other locations where we use Sonos (2 separate bars), and no issue there, only the one in the restaurant.

Any idea what is happening?