I'm having the exact same issue. The picture in picture is invisible. It exists and you can interact with it but you literally cant see it.
It works just fine in other browsers.

The annoying part is when you change tabs from YouTube to something else and the invisible picture-in-picture pops out and stops you from clicking anything that happens to be behind it.
You then have to go back to the YouTube tab and disable it manually.

If anyone knows how this can be fixed please share. Thanks

@leocg Thank You! That actually worked!
The option I enabled is called:
"Use graphics acceleration when available"
and I searched for it in the search box in Opera's settings then enabled it and restarted the browser.

I'm having the exact same issue. The picture in picture is invisible. It exists and you can interact with it but you literally cant see it.
It works just fine in other browsers.

The annoying part is when you change tabs from YouTube to something else and the invisible picture-in-picture pops out and stops you from clicking anything that happens to be behind it.
You then have to go back to the YouTube tab and disable it manually.

If anyone knows how this can be fixed please share. Thanks