Thanks for that. I experimented with it and when the account that owner/created the portal was deleted the creator and owner changed to 'Portal Owner', and everything kept working without doing anything about the App Reg.
I can't find any docs on this behaviour, so it's impossible to understand what to expect.
Also, what happens with the ownership when a portal is deployed using pipelines? Is it owned by the service principal that deployed the page instead of Portal Owner?
Ultimately we want to make 100% sure the portal will never go down if a user account is deleted, and I'm not sure what we are seeing with this Portal Owner behaviour.