Safari video viewer on macOS - sound level decreases by itself

Open Safari, go to Youtube and start video playback. Click on Video viewer:

Now, either minimize main window (to get Picture in Picture mode) or leave it like that and turn the volume up... after a short while or maybe a minute, volume drops down by itself...

I do use earphones at this point. And this is going on for a while now, not new to macOS 26.1.

In the recording below, this happened after a few seconds of watching... so annoying!