@giberie Long time ago when the PiP was introduced to newer Opera (post-Presto), seems like Opera had its own implementation of the PiP (no other Chromium-based browser had such feature at this time) they had PiP with caption (it looked like that they've embedded video player/stream into another floating windows so it had basically almost all features of normal player).
Later Chromium added a basic PiP function and Opera's dev switched to it. A Chromium PiP don't support captions and probably Opera's devs doesn't want to diverge the implementation (it's easier to maintain this than fixing bugs alone).
I'm using Opera for many many years now and unfortunately it seems like it's worse and worse browser to work with. They have many neat features which I like (messaging apps in side bar, workspaces, Flow, synchronisation, News feed) but some of them are not working well for many months/years (like synchronisation is a big mess with massive tabs chaos and multiple duplicated devices; Flow sometimes works, sometimes it log off and it's a problem to reconnect devices; random account disconnection which leads to lack of synchronisation). When PiP was introduced it was super with those captions but then they've switch to definitely a little worse implementation (at least from function point of view).
Sadly they're not listening to users and seems they're not intending not to.