@SafdarZaman Thank you very much for your reply. I agree, it seems that Opera overrides any personal preference with the official YouTube app. I would understand that if we were talking about Chrome, but Opera... why?
Settings → Apps → (Your preferred app) → Open by default → Open supported links
Links are disabled for YouTube and enabled for NewPipe.
Clear YouTube’s defaults
No impact.
Disable the YouTube app (temporary test)
When I disable YouTube, links open in Opera instead of NewPipe. I can only open them in NewPipe by opening the menu and select "Open in app". As showed in the video, Opera is aware of NewPipe being my default app, it just ignores it.
Try Opera’s site settings
By long-pressing a link, I can share it to NewPipe. It is the workaround I'm using right now, though it is a bit awkward.
As I said, I had no issues until version 86, so there is clearly a regression here that could be fixed.