Is this not a deeper problem @come-nc ?
So this method is not marked as throwing, but just to return null if not found.
Meaning should we fix the getOwner method or at least add the throwing to the docs?

This is how we manage non-existing files, see https://github.com/nextcloud/server/blob/master/lib/private/Files/Node/NonExistingFile.php

We could mark all methods as throwing but that would trigger a lot of false positive.

Actually for getOwner the throw comes from Node::getFileInfo directly I think.

But in this case the file exists

No it does not.
Look at the trace in nextcloud/forms#2435
It’s triggered by rename hook, so either source or target does not exist, depending if we are in the pre- or post- rename hook/event.