Yet another weird problem: For some reason, Canvas has marked all homework, except the first, with submissions on paper, as having TurItIn enabled. The problem is that if I choose submission type as On Paper, then there does not seem to be place for changing this. Only online submissions have that option.
One possible explanation: After creating the first homework, I switch categories to add a short history paper that did have TurnItIn enabled. Then I returned to homework and started working on it. But this is just silly.