What Vivaldi shows me:
What firefox shows me (this is what I expect 
):
Piratebay is blocked in the UK, but apparently vivaldi is ignoring it
Can’t see anything in the settings…
What extensions are you using in Vivaldi?
Nothing that will affect this, just the usual ad blockers and a few utilities. BTW this is snapshot, not stable, stable blocks it.
Did they implement DoH in snapshot?
No:
Interesting results you’re getting. However, I use VV stable as my daily driver, and I can prove that it doesn’t have a VPN installed. I use OPENDNS on my router/network to handle the web’s nastier places, so that my kids are not smothered in porn. I cannot get to a porn site through VV. But (and don’t tell the kids!) if I open Opera and use it’s “vpn” it is possible to get to a porn site.
Yeh, really weird, I meant to post of the vivaldi forum about it but forgot! Always tomorrow 
I have read somewhere - albeit long time ago - that vivaldi and opera - the opera devs sold out to China and started vivaldi - is offering some kind of proxying as to speed up mobile connections.
Maybe what you have experienced is such proxy.
Potentially. I will post on their forum, later, have a medical appointment. Would have done this yesterday, but it was too hot to think and I was trying to recover my emacs config after I converted to the straight package manager, but am up and running again